Of course, one big problem with a direct follow-on would have been that a year or two back the BBC Trust redefined CBBC's 'mission statement' to aim it at a younger audience: 8 to 12s, rather than 8 to 16s as before.
That left SJA a bit too 'adult' for the channel's remit, but as an existing series it was allowed to carry on as was. But if you watch the actual replacement, Wizards Vs Aliens, it's noticeably more childish (a bit too childish to be the proverbial 'Children's series that's good enough for adults). Though having said that, the last episode of season one declaring that they're not playing games any more, implying that the production team want to take it in a more serious direction next year.