When the original Walking Dead first premiered on AMC the first "season" was only 6 episodes. I figured they wanted to see if it would fly as a regular show and if the fans of the books would transfer the loyalty to the show. Both of those things happened, and now the season for TWD is usually 16 episodes (I think). Most shows on "cable" have shorter seasons, I've noticed. Perhaps it is the shorter attention span of audiences nowadays.
I saw an interview awhile back of a showrunner for IIRC Breaking Bad. The interviewer asked why they did not opt for a 20 plus show season like the alphabet Networks. His answer was very direct. It was something like "We have x number of hours and x number of dollars to get a season done. We want to put out the best product we can with the highest quality possible yet keep a realistic schedule for shooting. Bottom line we would rather produce 7 killer episodes than shoot 4 pretty good ones and the rest be subpar."
And being Breaking Bad, The Walking Dead, Mad Men, Justified etc. all tend to be waaaay better shows than most anything on the Alphabet Networks and have way fewer shows per season I feel they must be doing it right.