Some television channels have laws, or at least rules, that commercials played during an animated series cannot directly promote toys based on the series itself. The viewer is more likely to see such commercials played in a similar time slot during some other show. This does not always relate to other kinds of promotions, such as contests.
This probably results from media watchdog pressure during the early nineties, when parents became sick of the blatant Merchandise-Driven aspects of shows their kids were watching (which parents pejoratively referred to as “half hour commercials.”) Many shows during this period, like Transformers Generation 1 and He-Man and the Masters of the Universe, were actually conceived alongside the toy line solely as tools to promote toy sales, as opposed to more innocent times when toys were created to boost show ratings. It certainly does not affect programs for older audiences, which at least manage to hawk their newly released DVD volumes.
This extends to showing real products in the show itself, which, interestingly, in Yu-Gi-Oh!, led to all the text being wiped off the cards in the American version so they don't look like the real ones.
At least in the US…
This trope is actually a law enforced by the FCC. A show can't have ads for its own swag in its commercial breaks, or have blatantly obvious references to its own merchandise in the show (such as Yu-Gi-Oh cards with the real text on them). While it's a big advertising loss for the show, it is less obnoxious for the parents, at least.
And in the UK…
Early in TV's life, the UK broadcast regulator forbade adverts which featured actors also seen in the programme the advert broadcasted during. This was later dropped following the rise of multi-channel television and channels purely showing syndicated content.