Its complex Rooinek, sorry if you find this along read.
Whats called the Irish Backstop has been cited as the primary disagreement preventing the deal. I can't say for certain but its possible the withdrawal agreement would have been passed by parliament already if not for the backstop. But its difficult to say for certain because there is many in the Conservative Party that dislike the withdrawal agreement in its entirety not just the backstop as they feel the agreement still ties them to the EU too much.
Currently because both Ireland the UK are EU members they have regulatory alignment, meaning goods can pass freely from NI to Ireland and vice versa with no checks. As it stands its an invisible border. The UK leaving changes that. While the UK, Ireland and the EU do not want a border, a border will be required to protect the integrity of the EU single market (and the new UK market). Basically the EU are saying they can't allow a back door into the EU where smugglers could move goods from NI into Ireland or vice versa without the relevant customs charges, paperwork and tariffs being applied. Otherwise everyone trading with the EU will say hey that's not a level playing field, your allowing tariff free goods into the EU via the NI border but are charging us tariffs. So a border is required, but at the same time the EU have acknowledged the special circumstances of Northern Ireland and its relationship to the Republic of Ireland that predate the EU.
In brief the backstop is the EU and Ireland's position that if the UK leaves and no arrangements are found to prevent a hard border returning to the island or Ireland, then Northern Ireland should remain in the EUs customs union until an arrangement is found that would not require the return of a hard border. This is in order to preserve the spirit of the Good Friday Agreement the treaty that brought an end to the 30 year long conflict in Northern Ireland known as the troubles. Its also for economic reasons, a border between north and south would cause damage to north south trade.
The British proposed an electronic border as a solution to the NI border issue. Which would involve camera's that could read number plates, per-approved trader schemes that would see all paperwork and customs check done away from border at the point of origin or delivery.
But the Irish and EU rejected that on the grounds that the technology is very expensive, the pre-approved trader schemes were not financially viable for the majority of small to medium size business that use the border and most of all the solution still required border infrastructure and would likely be a target by nationalist paramilitaries. Not to mention the technology does not exist and the British government admitted that saying it won't be ready till 2030 at the earliest. Think about it for a minute, your going need camera's that can read number plates at night or in heavy rain or fog, do you know of technology that can do that?
The UK has not come up with any other arrangements, though various Brexiteers have proposed the same technology border soltuion in slightly different wording several times.
If NI remained in the customs union, no border is needed. Checks of products going to Northern Ireland from the UK could be done in British ports and airports and that would be considerly far less intrusive than a hard border in Northern Ireland.
But there is have a problem with this. And this goes back to the last general election in the UK. What happened is when Theresa May called a snap general election, the Conservatives had the largest single share of the vote but still lost their majority in the House of Commons and they needed the assistance of another political party to form a majority in the House of Commons to be able to govern. That party was the Democratic Unionist Party from Northern Ireland the largest single party in NI but small in the overall political landscape of the UK. The DUP are an ultra hardline unionist party and Brexit supporters. And when I say ultra hardline unionist I mean ultra ultra ultra hardline. Anything they perceive as a threat to Northern Ireland's place within the UK will be nuked from orbit so to speak and that includes Brexit. A side note is they are also the only political party in NI that oppose the Good Friday Agreement.
So to them the backstop which would leave Northern Ireland in the customs union is basically in their eyes a lessening of the union and is completely off the table. They will bring down the government over it. So in effect the DUP has yielded massive influence over Brexit out of all proportion to the 10 MP's they have in parliament. So the EU said fine, how about we let the whole of the UK remain in the EU to placate the DUP's fears and ensure no difference between NI and the UK, but the Brexiteers freaked out at believing this concession was a trap to keep the UK in the EU as the EU would have to approve of any alternate arrangements giving them ultimate power over the UK leaving the customs union. The DUP and other Conservatives following their lead cited the backstop as a threat to the union and voted against the withdrawal agreement that included the backstop on 3 occasions. Though many suspect that some of the hardline brexiteers in the Conservatives really wanted no deal all along and used the backstop as an excuse to reject the withdrawal agreements.
So as it stands the Conservatives and DUP have a parliamentary majority of just 1. All other parties are united against a no deal exit. All it takes is for 1 conservative MP to rebel and no deal is blocked. Most estimates put the number of conservative rebels MP at somewhere between 10-20. They believe so strongly that a no deal brexit will be so bad for the UK that they will bring down their own government.
The UK government's position under Boris Johnson, is they want a trade deal with the EU but they want the withdrawal agreement re-opened and the backstop removed. The backstop has to go before the UK will even go to the negotiating table. They want the Northern Ireland border issue settled outside of the withdrawal agreement and in effect to basically trust them to come up with a fix in time. If those terms are not met, the UK will leave with no deal, though whether he can do this against parliaments wishes is another question.
The EU/Irelands position is the matter has already been settled. The backstop cannot be removed. They have the only known fix to the NI border issue. Its not perfect but its the least disruptive to the GFA and the north south economy. The Irish also do no trust the UK when it comes to the NI border and their promises to sort it out later. They promised the border was only ever meant to be a temporary arrangement when Ireland gained independence from the UK. Now they are breaking the spirit of the GFA in Irelands view. They view the UK's unwillingness to sign up to the backstop as an acknowledgement that the UK doesn't have other working alternate solutions to the border issue.
So the stare down between the UK and EU continues.