The title basically. I was gone for a while and I'm just trying to catch back up on whats what with league and its champions. Especially Elise, when I left she was considered pick or ban and now shes trash tier? What gives?
She's just been nerfed to the point that everything she does, someone else does better. Her early game execute potential isn't as great in this season as games are lasting much longer and tanks are the big thing.
Because being able to duel 
in the early game is no longer a pick or ban worthy trait in a jungler,
Elise received several nerfs to her Q dmg and was notoriously picked for her ability to tower dive and 2v1 the enemy top laner at lvl 3. The meta has drastically shifted with all the changes made at the beginning of season 5 and this current meta heavily favors strong teamfighting ability to secure dragons and barons. Elise's skillkit makes her a weaker pick against teamfighting champions that provide alot more AoE abilities.
In the second half of S4 (at which point she'd been pick/ban in LCS for over a year) she received several nerfs that took her from top-tier to just okay.
Then the S5 jungle changes completely screwed her. Squishy AP assassin junglers in general aren't doing that great (see also: Diana, and nobody's running Akali jungle anymore).
Jungle itemization changes hurt her a lot.
-No more tenacity on Golem was actually a big deal
-Cinderhulk just isn't a good upgrade for Elise. She has the base stats of a ranged champion and health stacking with low resistances is kind of meh.
-Magus is pretty trash compared to Cinderhulk in general.
Other than that I think they screwed with her rappel mechanic. Like she doesn't get the free auto/Q straight out of her rappel anymore. Also a pretty big deal and makes her early game pretty average when she used to be the queen of level 3 tower dives.
