The whole secrecy thing made some kind of sense on CCS, but now... really, it'd have been a consolation to them. Well, maybe he was scared they'd help him but he didn't stop S&S from getting involved.
Wait. If Clow knew S&S would get involved, then not telling the Guardians makes even less sense. Because they'd probably agree to stay and help them once...
But I don't even remember which S&S are they supposed to be, so.