Jim raises a good point. I think also, the different niche's of riding are ever changing. There was a time everyone thought a bike for what we call 'urban assault' today should have no shocks and slicks. Today they're about as beefy as DH bikes. Mountain biking in general is always changing. You look at forks and frames from but a few years ago and you see some pretty strange things.
I'm sure there is a trendiness factor too it though. Like the OnePointFive standard Manitou started a while back. People found that it was a fairly good idea, and looked kinda nifty too. Today we're seeing bikes with 1.5 head tubes that really shouldn't have them, XC bikes for example.
The fact is, companies will do whatever they think will make more people buy their bikes. Its a business after all.