He's constantly hailed "great" by critics and hardcore fans when in reality I get the impression most people didn't like TPAB and probably don't think GKMC is miles ahead of [insert whatever other great rap album has come out in last decade]. He has zero radio presence outside of guest appearances (most of which are BAD btw, as seen by the fact that his verse is entirely cut from Goosebumps which is a hit record now). He doesn't tour outside of festivals, which suggests he's not able to sell out major venues on his own.
And in general rap has moved on. This current atmosphere is all about Future, Thug, Kanye, etc. This is a post Pablo era, no one has time for what Kendrick is doing.
I'd go further and say Big Sean could probably take him in a battle. Sean's verses are concise and make sense, and he has punchlines. That matters in a rap battle. Kendrick hasn't had a QUOTABLE verse since Fucking Problems.