Anyone know...

why Aiden Markram hasn't been added to the ODI squad?

no

 I'd like to see him in the ODI squad, as an opener he would change the dynamics of the team. For starters Amla would anchor the #3 slot which would be appropriate at this point of his career, AB @4 and Faf @5....totsiens  Duminy. My team going forward would be...
1) Markram2) De Kock3) Amla4) AB5) Faf6) Miller7) Philakwayo8) Rabada9) Morkel10) Philander/ Steyn/ Ngidi(spelling?)11) Maharaj

 @Denny
Why would you break up the best ODI opening partnership in the world?

Markram will get his turn.

 And as i log on to Cricinfo and see this:

South Africa opener Hashim Amla has been rested for the final ODI against Bangladesh on Sunday.

He has been replaced in the squad by Aiden Markram.Amla, 34, was given a break after he scored a century in each of the two Tests against Bangladesh, and 110 not out and 85 in the first two ODIs of the three-match series. South Africa won the Tests 2-0 and have already taken a series-winning 2-0 lead in the ODIs.Markram, 23, had made his Test debut in the two-match series against Bangladesh, scoring 97, 15 and 143 in his first three innings.

He then made 82 for a Cricket South Africa XI in a 50-over warm-up game against Bangladesh, and is now set to make his limited-overs debut for South Africa in the third ODI at Buffalo Park in East London.

Time to experiment!

Amla has his critics on the Board, but they are quiet of late. I hope the youngster comes through....what I see of the new bowlers is not all that positive.

 I am not favouring the break-up of the opening partnership - but the gradual  introduction of younger players to avoid what happened to our rugby after the 2015 RWC.
This is where I think that Markram should start playing gradually - especially as far as the Bangla Desh ODI's are concerned.    To my mind they missed the boat here selecting Behardien and Duminy - who are way over the hill - instead of younger players.  
I think there is talent amongst our bowlers - players like Rabada, Mahataj  and even Pretorius - and hope that a player like Mulder would come through as well.   It is not enough -  and I would rather want to see a few more.       

Well I was not really thinking about Rabada, who is a proven test performer. That said he has lost a bit of his youthful hubris and the sledding is a bit tougher as he gets to grips with a long term career.

 Why would you break up the best ODI opening partnership in the world?
FlashD, going by your post which follows the above  it's safe to assume the selectors have very smartly decided to explore the potential of using Markram as an opener in ODI's going forward. 
Apart from the reason I've already stated there are several other reasons why it may prove to be an excellent idea, one that with a bit of luck could eventuate into WC glory in two years time.
I should say that your question opens up a very interesting debate although I hasten to add that for mine it's a no-brainer.
And Moz, you're perfectly correct, the new brigade of quicks i.e Paterson and Hendricks as well as the spinner Shamsi leave a lot to be desired, they are not of test standard.
There's talk of giving the new board sensation Wiaam Mulder a run in today's 3rd ODI....here's hoping the Proteas bat first so I can see him for the first time.