Some platforms do have a choice of compilers.
Also, some compilers optimize better than others. Supported compilers are ones that Athena believes will work well, that someone is willing to take bug reports about, and that you can get some help with. The gcc compiler is fully ANSI compliant, and with a variety of command line switches one can vary how strict it is. This compiler has a good optimizer, but it is slow and its parser can become confused. The other two invocations are much stricter and are often difficult to use.