Forum OpenACS Q&A: Response to Future directions for OpenACS
Now, it's true that there are probably more in-house projects using Java (since it's more in vouge) than Tcl, so ACS Java meets a larger portion of the folks with existing infrastructure who don't want to learn anything new than Tcl would.
Personally, I hope development of ACS Tcl continues. The "quote of the week" on the comp.lang.tcl newsgroup this past week was: "I've come to the realization that there are so many more Java jobs than Tcl jobs because it takes 10 Java programmers to do what I can do in Tcl." There are folks out there with Tcl web/db experience--it's just that a lot of times they call themselves Vingette developers.