Actually, there was a reason. It was replaced by Fairly OddParents because the 2nd half of the show had mentions of Ronald Reagan in it, and he just died, so Nick thought it wasn't a good idea to air that half of the show. Why they didn't replace the whole show with another You Can't is beyond me...
But the thing is that they never mentioned this reason on-air. After the break, they went to Oddparents, with no explanation to the viewers. In the end, the backsell ended up being awkward, with the host closing OddParents with "That was your Old School pick" (the "Live" segments were actually pre-recorded that week).
This incident wouldn't have happened in the first place if Nick screened their shows in advance.
As for replacing it with another episode, I think Nick only had the rights to show 2 or 3 episodes of YCDTOTV -- if it was "3", they could've put that one on; if it was "2", they had to show the previously-picked one again. If they wanted to show any other episodes, they had to contact CJOH in Ottawa first (they produced and own YCDTOTV), to procure the rights. But of course, Nick is too lazy to do anything right.