Because Java-only solution to a complicated problem is normally simpler.
Few reasons contribute to that. One of them is exhaustive pure Java library ecosystem, which means that it's very unlikely that you'll run into intricacies of native library access on your host system. Another is excellent tooling, which means that when all your processes are Java processes, lots of operations-related tasks (process management & monitoring, log rotation, etc) get significantly easier.
Indeed, pure Java crypto libraries (Bouncy Castle) were really nice a few weeks ago. When other languages/frameworks are wrapping or calling openssl, it's a really nice feeling to have a common crypto algorithms implemented in your languge/platform. Same goes for image encoding/decoding, database access, lots of examples.
Plus, there's the benefit of (essentially) standard builds, project layout, dependency management, and deployment strategies (jar or war). A java developer can walk into a new job and be productive right away.
Few reasons contribute to that. One of them is exhaustive pure Java library ecosystem, which means that it's very unlikely that you'll run into intricacies of native library access on your host system. Another is excellent tooling, which means that when all your processes are Java processes, lots of operations-related tasks (process management & monitoring, log rotation, etc) get significantly easier.
More on that I wrote few weeks ago: http://www.mikhanov.com/2014/03/31/a-love-letter-to-java-363