AP hit it directly on the head..after a while, the casino sees that it cannot make money on you (in their eyes) because your play hasn't changed at all.
Ultimately (this was told to me by a long time SENIOR host) what the casino wants is for you to keep raising your playing level, and they will keep giving you comps (kind of like putting a carrot in front of a horse, to entice it to keep moving). Once they realize you've "plateaued" they will pull-back (remove the carrot) and move on to other customers...it's a simple business model really.
The only people who wouldn't have followed that have all since got out of the business (Benny Binion, Jackie Gaughn, John Ascuaga)
To avoid this, somehow get ahold of a "new account" and start off with say $10k coin-in, then next trip move up to $11k coin-in, and so on and so on...watch what happens.
(I actually did this, except I was more strict, I did $10,005, then $10,010. LOL)