Why exactly is Evelynn getting a rework?

Because her kit is old, incredibly outdated and quite awful. Plus every other jungler craps on her in this meta. Her damage output can be mirrored along the lines of supports, and the list goes on.

source?

Her kit is incredible dated.

When touched with balancing attempts, she's either completely trash or destructively OP due to how hard it is to balance stealth, which is her biggest point. Plus, her kit's a bit too old.