Another reason is that the JVM is highly optimized, well-evolved, and extremely complete ecosystem. On it's own, it competes extremely well with any of the other compiled languages. (I won't say that it's the best general purpose VM out there, but I've certainly banked my career on that.) So getting access to the JVM, short of writing bytecode, is desirable in itself.
However, the Python VM is good, but (nothing against Python) has some serious shortcomings. The Python runtime environment suits the dynamic nature of the language well, but can really surprise you when you get familiar with its memory usage, global locking, or threading model.
In head-to-head comparisons, the JVM is typically twice as fast as the Python VM. The JVM (surprizingly) even competes well with natively compiled code, based on the "hot" optimizations it performs. And that's not even counting the more sophisticated thread handling, etc.
I love Python, I really do, and hate to say it, but sometimes the performance just kicks me in the teeth -- otherwise, why would critical Python libraries like numpy or scipy have to fall back into C code?
In other words, people who gravitate to Python do so because they like the language. But if you want to write a brand new language to suit your preferences, you're much better off compiling to the JVM, because your new idiosyncratic language will start off in one of the best (subjectively, maybe the best) operating environments available.