There seem to be no spells that cost XP, creating magic items doesn't cost XP, and monsters don't drain levels. The first two, at least, were significant considerations in 3.5. I can speculate, but do we have any word from the designers on the reason for the change?
The designers put their reasoning right in the DM Guide. Experience Points are now optional to help make the game work with more styles of play.
[Doing] away with experience points entirely...can be particularly helpful if your campaign doesn't include much combat, or includes so much combat that tracking XP becomes tiresome.
(DM Guide, page 261)
To support playing without experience points, you can't have a mechanic that requires them, or an economy that uses them.
It's not a complete answer, but according to Mike Mearls (head of the D&D 5 R&D team)
I don't track XP. I let the PCs level up after every other session or at the end of each key adventure.
That may give you some small insight to the designers thoughts on the matter.