It is because Diamond tables play with much more consistency and the valleys do not, even with the ridgeback rails. I know, I've got one of each in my basement and I upgraded the valley with the Ridgeback rails (which are awesome and I heartily recommend, by the way). Many valleys are just old and beat down in many/most locations and they just have to many bad rolls due to bowed/sanded/whatever else slate, poor cloth conditions, poor balls, improper rail height, etc, etc. The list goes on and on. Anything that is inconsistent has a tendency to shark players, even good/great ones.
For instance, one can and should play a finesse game on proper equipment. In such an environment, you can slow roll balls and have them roll straight. Not so on many of these valley/other bar tables. The result is sharking the shooter because they can't play their normal game. To avoid such a poor result in the bars with sub-par equipment, IMO, you should put a little more "pepper" on your shots and avoid slow rolls almost at all costs. Just my opinion of course.