I think they kept alignment for various reasons, some of which are outlined above, but also for 4e in particular the following seem to be emphasized:
a) A metagame conceit to try to keep certain players from being dicks by discouraging them from playing characters more likely to be dickish. And even if you are among the few who can roleplay evil without annoying other party members, most people new to the game who try evil characters usually play Evil Annoying.
b) To show demonstratively that the good guys don't always agree, but can usually get along somehow (LG, G).
c) To show that there is Evil (E) and then there is Batshit Crazy Psycho Evil that doesn't think of the consequences (CE). Gruumsh likes destruction (and if everything is destroyed, then what?). Lolth actively works against the interest of most of "her" people, the Drow (who basically are trying to organize, but are set up by their priesthood to fight each other at least as much as any other enemies). A Certain Other CE God (and all demons) wants nothing less than to destroy Everything, including perhaps Itself. Thus you can sometimes, unwillingly, be forced into an alliance of convenience with some Evil entities, but there are others that are so far beyond the pale that we are talking the Reavers from Firefly/Serenity.
d) To show that most gods/people don't play the alignment game, but rather have their own agendas perpendicular to alignment concerns (Erathis vs. Melora, for example).
Other than that, alignment has been reduced greatly in influence in 4e. 1st ed AD&D had actual alignment languages. 3rd ed D&D had creatures literally made of Evil or Good and/or Law or Chaos in the way that people are made of Matter/Energy in the real world.
I personally think they have struck the right balance. For those who like alignment, it is still present in some form, and could be expanded. For those that don't like alignment it drops out sooooo easily. Just replace Cleric and Paladin starting alignments with the specific commands of the god(s) they are devoted to, which commands are handily associated with each god. Nothing else has to be done.