Why do Tom and Daisy decide to stay with each other in the end?What's keeping Tom and Daisy together, since they both have enough money that they...

A part of the reason, at least, is because they share a child together.  There was at this time, even in Tom and Daisy's social set, a strong prejudice against divorced couples; especially couples with children.  Daisy, especially, would have risked serious social scorn.  Even though Tom kept a mistress, if he were discreet enough the prevailing idea among their friends would be that they should avoid divorce at all costs. 

Another reason is that girls like Daisy "didn't marry poor boys," as she had said all those years ago in St. Louis.  Gatsby, though he had made piles of money, was still the social parvenu -- the nouveau riche.  For Daisy, even if she loved Gatsby still (or ever, and that is debatable), divorcing Tom and marrying Gatsby would be a step down for her socially.  Again, the opinion of others would be paramount to Daisy; it was the only social currency and standard of worth she understood.

Gatsy made several blunders in regards to Daisy, however, that didn't help his already hopeless situation.  He spent money like the new rich often do: with obscene abandon.  His overly lavish lifestyle, and, especially, his parties full of parasites and dubious hangers-on, would have lessened Gatsby's social status among Tom and Daisy's friends.  And Tom and Daisy's friends' opinions were what mattered, ultimately, to Daisy.  Gatsby also told some exaggerations and some outright lies about his past, and tried very hard to be something he was not (a product of a wealthy family).  If Gatsby were to have any chance with Daisy (and, again, that is debatable) he would much better have owned up to his past and been proud of his humble origins. 

But Gatsby was a bootlegger, and lived by criminal activity.  It was in his nature to lie and to cheat, and he thought by wishing and saying enough that he had a similar background to the man Daisy did marry, Tom, that it would make it so.  Gatsby found out, too late, that this was never going to be the case.

Also, Gatsby was far too intense in his love, which was almost worship, of Daisy.  This, understandably, frightened Daisy enough that her philandering, blowhard husband -- the "devil she knew" -- was preferable to the volatile, lying, criminal Gatsby.  In the end it didn't matter how charming Gatsby was; he was not the man Daisy would choose.