It goes like this:
- Studio makes TV series A, B, and C.
- A turns out to be really good, whereas B and C are the usual pap.
- Public realises that series A is really really good and it makes series B and C look like the worthless crap they are.
- Ratings for series B and C plummit, public starts expecting Studio to continue putting out shows on the level of A.
In order to cut off point 3, and therefore maintain the viability of series B and C, and ensure that they can continue to produce worthless pap for eternity, Studio gives series A the chop.
Alternately it could just be that all the really crap series blur into one, and we therefore only notice when the good ones get canned.