I think Kvetch is right.
Apparently the code only permits a set number of skills/spells, and we have that maxed already.
(I think the number is 200, but I might be wrong there. Any of the coders can probably answer that).
Anyhow, that's why Mord started with the subskills, some years ago. Just raising the number of skills/spells allowed was not an easy operation, I gathered, or he'd have done that instead.
Actually I don't think it's a bad idea to substitute some skills/spells with new ones, if you can think out some that actually ARE better and/or more interesting. I bet there are a number of the current skills/spells that nobody ever uses, and having thousands off skills/spells might sound cool in theory, but only gets confusing in practice.
It's a bit like having hundreds of races and classes. (Only Twink muds do that, because the differences are really just cosmetic, there is no way you can make them all significantly different and still keep some sense of balance).