DISCLAIMER: I like the simpsons.
As I am not famiiar with the American television that much, I can only speculate.
If a show gets cancelled, it doesnt make enough money, thus it doesnt have enough viewers.
If a show continues, it keeps making enough money to be continued, thus it has enough viewers.
Companies want money, right?
Earning money is more important than being a dick, in their opinion.
Because FOX keeps Simpsons on, it has viewers. It is popular.
Because FOX cancelled, for example, this Firefly show, it didn't make money = it didn't have enough viewers.
I understand that the airing time, developing costs and everything else affects this, but still. If one show has more viewers than the other, it is more popular. Debate about a show being better than others is based on the personal opinion of a small group of inviduals, but here the numbers show the truth, in my opinion.