Why not 80? Why 50 to 60? Is it because WoW started off with 60 as the level cap in vanilla, and is doing the same thing with D3? In D&D rpg's 20 used to be the level cap, and in 4th edition it became 30. In most other rpg's 99 or 100 was a generally well accepted level cap. What is wrong with 80?
Ehhh the more I think about it the more I think they just put two factors together (horribly oversimplified, of course):
1) We want leveling to take X time (both in terms of getting 1 level up and the entire journey)
2) We want you to go through Y dynamic event and/or Z personal story content before level capping
One reason to have 80 levels might be that they want everyone to be able to experience every zone while leveling AND be able to PvP some, do some WvWvW at the same time. So you don't level to fast and won't be able to experience every zone as you're leveling.
Some guys, probably a lot WoW fans that'll try it out, are gonna do like they do with WoW. Rush the continent and play "end-game" (for them) which is PvP, Elite DE's, Dungeons and WvWvW, they won't be experiencing all zones and all that if it's to few levels maybe. Idk, just a shot into thin air