The fact is Stephanopoulos is a good communicator, he knows politics as much as anyone inside the Beltway, and he came along at the right time or the job.
He has generic, acceptable skills-any number of people could do his job at least as well, save for the supposed advantages of Clinton exposure.

It's worth discussing because (1) he's a skin-crawlingly unlikable twit if you know the history, which interferes with watching shows involving him, (2) he has no integrity, which is supposed to be the centerpiece of journalism, (3) he still comes across like a kid lacking depth-others are better, and (4) he's only there because of Clinton, the guy he thanked by later attacking like a rabid dog. Others have done it, aren't on TV as a result of it.
Don't assume networks know what they're doing all the time-his selection was about as valid as Katie Couric's.