Most cartoon have a minimal number of episodes per season (10-16 being the usual number) that means within the span of a year each episode can be shown at least three times or more if the cartoon is aired multiple times on the day which it airs. That means that within several months time, a more than casual viewer can see all of the episodes at least once. A frequent viewer can see them multiple times.
Given the brevity of animated series ( almost all 30 minutes (22 minutes + 8 minutes of advertising) and the need for networks to air multiple programs as well as films of varying lengths, there is often a need to adjust the schedules accordingly. A cartoon in heavy rotation or which is not carrying its usual air time ratings (cartoons, like live action programs are also affected by viewer ratings) are often pulled and replaced by a program which the network may think will attract viewers.
Finally, given the limited number of cartoons in existence and the fact that cartoons from the 1960s/1970s are no longer in many networks rotations,to keep viewers "interested" changes in the rotation have to be made. The network again has to program to make rating and attract advertisers, so airing the same cartoons all of the time may not be what will make this possible.