Java doesn't scale to small projects and isn't "highly performant" when the metric you're trying to optimize is page load time, although it could conceivably be made to "support in-browser scripting".
Also, I think an implicit item on the list would be "doesn't suck", which is the whole point of the effort, not merely a constraint on feasible solutions like the "goals" you listed. They want something that sucks less than JavaScript, not something that sucks more, such as Java.
So I don't think "the reasons not to basically boil down to NIH, politics and the encumbrances"; they boil down to Java being a shitty non-solution to their problem.
I understand that you don't seem to like Java, but it is really immaterial to my point. They stated all the goals of Java in their feature list and then didn't mention Java. If they thought Java sucked or didn't meet their requirements even though it has the same stated goals then they might have said so. But they didn't say anything at all - it's kind of an odd omission to make by accident.
Also, I think an implicit item on the list would be "doesn't suck", which is the whole point of the effort, not merely a constraint on feasible solutions like the "goals" you listed. They want something that sucks less than JavaScript, not something that sucks more, such as Java.
So I don't think "the reasons not to basically boil down to NIH, politics and the encumbrances"; they boil down to Java being a shitty non-solution to their problem.