He's always been niche. Not really in the same ball park. I'm not the biggest Nick Cave fan by any means, but I don't associate him with reinventing himself constantly the same way I see Bowie doing that.
Does being niche matter? And while he hasn't reinvented his music as often, possibly, he's certainly done a better job of it when he has. Bowie from the early 80's all the way up to Reality in 2003 is pretty bad, Cave has only really done the one bad album (subjectively speaking of course).