Usually with some programmable piece of HW (and not only a bare mcu) it is easier (although not trivial) to build the HW itself than to write the appropriate SW. Especially so, if the SW is extensive, has to fulfill various requirements of many users, and has considerable degree of freedom in details of implementation.
Compilers and other development tools are such piece of SW. And it is not fun, rather hard work, to make and maintain a compiler.
GCC is undoubtely responsible for at least part of success of ARM, AVR and (I was told) 68HC12. It is not only gcc is free, it is also open source. There is a lot of people continually improving it and in many ways all the part-specific ports benefit from this huge background. The concept inspires other people in creating the other tools (opensource and free, too), e.g. IDEs, simulators, debugging tools etc.
But - and there is definitively a "but" - the part to be supported/ported to, has to fulfil a couple of requirements, to conform in some way to some basic model/assumption. The '51 does not, yet. But why not grasp on the opportunity?

Jan Waclawek