GCC/G++ existed at the time but GCC didn't and still doesn't contain a RISC OS WIMP library, nor did it (and still doesn't) come with an IDE. I agree that neither is the purpose of the compiler but both are there for increased productivity. From what I read at the time - this was what Sibelius and CC were looking for and felt that available solutions didn't have. Of course there have probably been other issues as well but these were named in interviews and articles in (if I remember correctly) The Micro User.
Now there at least is the RISC OS Toolkit but wether or not that is usable I don't know but it looks promising.
Oh, and what IDE is it that you're referring to? I'm curious