 There were a few reasons the older guys were putting it down. Remember, many of the established studio musicians were from the old school big bands, and they were busy working in lucrative careers in soundtracks. When the labels started pushing some of the younger acts, they would create demos first. Now, the older musicians wanted to take a chance on taking over a demo session because it was illegal in the views of the musician union. Why take a chance when you're working on a movie for a three hour gig that paid less?