Good question - though there are a few GOOD rock bands around (Avenged Sevenfold comes to mind!)
I tend to think studios got lazy and greedy. Once rock musicians "paid their dues" by learning their craft, honing their skills and developing their voices and writing original songs. Then along came disco -which was basically vocals plastered in front of a simple electronic harmony. Cheaper than paying real musicians. Then came rap. The rap artists didn't have much work cut out for them as long as they could rhyme a few lyrics. And again, no musicians to pay. Today we have hip-hop, rap's disco-ized bastard child and a whole bunch of talentless egotistical "artists" that don't care if their music is uninspired cardboard - as long as they can look pretty to the plastic generation that has been raised not knowing what real music is.
Just like blues, jazz, soul and old school c/w, Rock was real. But the studios wanted the easy and quick way to make money. So they sold out (remember when selling out was a BAD THING?) and today real musicians on the charts are few and far between. It sucks, it truly does.