Blame the digital age. Blame China. Blame Eastern Europe. Blame Russia. It may not be right but artists are competing in a global economy and with the rise of the digital medium, the payouts have plummeted. I have a friend who is very well established in the business and has been for 20+ years, and he says there is no way he could start out and duplicate his success in this time.
Not the same medium, but photography is the same way. 10-ish years ago you could make some pretty good money with Stock Photography. Nowendays, with everyone and their phone thinking they are a 'photographer', you're lucky to make a hundred bucks a quarter. My stuff on Getty gets me enough to buy a pizza and some film every other month.
Basically, Supply has gone up, Demand has either gone down or stayed relatively flat, so prices go down. Also with more stock footage out there, it is harder to get eyes on any one given piece of stock, decreasing the likelihood of someone even finding it let alone buying it.
I've used a number of stock photography sites for filler material for stuff (presentations, papers, sites) and it is dirt cheap for several thousand photos that are then royalty free for use once I've downloaded them. So, those sites can't be paying the photographers much, if at all, given how cheap the monthly fees or the bundle download fees are.