With a lot of effort by many folks in the community, HBase has mostly tackled the full-GC problem, but still has occasional issues with some workloads.
So, GC was definitely one factor - not having GC means we can give 99th percentile numbers in the single-digit milliseconds, which is pretty nice. Our master process actually has shown <1ms 99.99th percentile for tablet location requests on an 80 node cluster. So again, numbers that are super difficult to get on the JVM unless you take an allocation-free approach like the HFT guys do.
Another factor was ease of integration of platform-specific code for performance reasons. For example, we make use of SSE prefetch instructions to improve scan speed in our concurrent B-tree by 30% or so. The b-tree itself would be difficult to implement in Java due to lack of control over object layout, etc. While you can eventually get the same performance with enough off-heaping and sun.misc.Unsafe, my feeling is that, by the time you've gone down that road, you might as well be using C++.
I'll admit that, after many years of not writing native code, I was a bit nervous of diving back in. Segfaults are never fun. But, we soon realized that the native code tooling has improved a _ton_ in the last decade. We run all of our tests precommit using the excellent Sanitizer tools from Google (ThreadSanitizer, AddressSanitizer, LeakSanitizer) and those make it nearly trivial to diagnose a leak or crash. We also have pretty strict guidelines around use of pointers, based on the Google C++ guidelines. Many will complain that this is a neutered form of C++, and they're right. But it's also a relatively safe form of C++.
I could probably write a lengthy blog post on our experiences of C++ vs Java, but hopefully the above gives you a taste. Overall I've been happy with the decision. Slightly more time spent on crashes. Less time spent on chasing hard-to-reproduce performance or memory consumption issues. And the thread checking tools are actually far superior, so I'd say less time spent chasing races.