For many people it probably boils down to which they like best, shows or rides, although many might not have thought about it that way.
MGM is very light on rides. If you consider the Studios Backlot Tour and the Great Movie Ride as tours - which they basically are - you are left with only three "rides" in the park:
(1) Star Tours, which is a simulator, (2) Tower of Terror, and (3) the Rock 'n' Roller Coaster.
On the other hand, if you like shows there are ten to pick from:
(1) Sounds Dangerous, (2) Indiana Jones, (3) Mupet Vision, (4) Lights, Motors, Action, Stunt Show, (5) Who Wants to be a Millionaire, (6) One Man's Dream, (7) Voyage of the Little Mermaid, (8) Playhouse Disney, (9) Beauty and the Beast, and (10) Fantasmic!.
So, it's one ride for every five shows. Add to that the fact that it is sometimes difficult to schedule a day in the park because of the wide range of show times.
Having said that (in an effort to see why some folks don't like MGM as well as the other parks), let me hasten to add that I personally love shows, and MGM has some of the very best at Walt Disney World.
Enjoy.