Because you can find libraries for all the "Enterprise" technologies easily. Besides .net, not many ecosystems offer that kind of library support. From XML signatures, to pkcs11, to complete soap support (including ws-security), to many others.

Fact is, in the Enterprise you'll have to interface with many of the aforementioned technologies, and Java still has the most support for them. We find that even in .net we sometimes have to implement some things on our own from the horribly convoluted "Enterprise" technologies because a specific feature isn't supported.