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?
23\$\begingroup\$ Historical note: While level loss (and losing experience as a consequence of level loss) was a part of D&D since its earliest editions, experience point costs for spells and magic item creation were only introduced in Third edition. They were dropped again in Fourth. \$\endgroup\$– GMJoeJun 24, 2016 at 3:50
7\$\begingroup\$ And also dropped in Pathfinder. \$\endgroup\$– mxyzplkJun 24, 2016 at 4:20
\$\begingroup\$ have you sent a tweet to any of the design team? \$\endgroup\$– KorvinStarmastJun 24, 2016 at 16:13
1\$\begingroup\$ I was assuming something this big would already have been in an interview somewhere. Also, I don't tweet. But if nothing turns up I may try it and self answer. \$\endgroup\$– SirTechSpecJun 24, 2016 at 17:01
5\$\begingroup\$ Posters, beware that purely speculative answers will be deleted, as they are not answers to the question. \$\endgroup\$– SevenSidedDieJun 29, 2016 at 17:40
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.
(From a Reddit AMA with Mike Mearls)
That may give you some small insight to the designers thoughts on the matter.