For a couple of reasons. One is that there's already a great Clojure environment for Eclipse - Counterclockwise. Secondly, I think that IntelliJ provides a better platform for building something like this - it's just a better IDE all round in my opinion, and it's a great infrastructure to build on.
I suspect that you'd probably find that the percentage of Java devs using IntelliJ has gone up a lot in recent years too, their market share and mindshare has increased massively recently. I'm not aware of any stats though (and I'm not sure how accurate they would be if there were). Google recently switched from Eclipse to IntelliJ for the official Android development environment, for whatever that's worth.
I suspect that you'd probably find that the percentage of Java devs using IntelliJ has gone up a lot in recent years too, their market share and mindshare has increased massively recently. I'm not aware of any stats though (and I'm not sure how accurate they would be if there were). Google recently switched from Eclipse to IntelliJ for the official Android development environment, for whatever that's worth.