Misconceptions about performance and poor JVM optimizations would be my guess. I say misconceptions about performance because there are some Java ports of C++ games that perform faster than their C++ counterparts (see Jake 2). The real problem, IMHO, is that many Java programmers aren't focused so much on bleeding edge performance as they are with ease of use and understandability/maintainability of code. On the C/C++ side of things you're essentially coding in a slightly higher level assembly language and its about as close to the hardware as you can get without writing in assembly or straight machine code.
