I think I agree with Robert and Stlhdfishn too.
It is a lot of factors.
One being that a generation (or 2) of people have been raised by parents who, themselves, don't give a crap about the environment (or anyone else) and just leave their trash behind thinking "someone" will pick it up.
Likewise, a hatchery run of fish brings out far more anglers than does a CnR fishery, ergo, more people, more trash.
Often, the leavers of the garbage are not fishers, but campers and gangs of kids partying on the river banks.
We had a group of 3 kids (maybe 17-18?) camp out on the beach across the river from our home on July 4th. They got in there about 11:00 p.m., and proceeded to start shooting bottle rockets across, and into the river, blowing up firecrackers till 2 am. I called the Sherriff, but this far up river it takes a while for them to get there (they never did), and Concrete cops aren't much help (unless your speeding).
These kids got up the next day about 9 am, took down their tents and the one guy that was there started tossing the garbage (big stuff) into the river. I shouted at him to take his damn garbage out with them and keep it out of the river, he gave me the finger and walked away. ( I wanted to drive over there and kick his butt...but then I would have been the one in trouble).
That area was clean before those kids camped there, and there have been dozens of people fishing that bank. I took a bag of their garbage out with me yesterday when I was over there stalking sockeye. There's at least another full bag of their garbage left...guess that will be taken out on my next trip.
Media (TV and music) today has taught these people to not respect anyone or anything, beginning with themselves. Their own personal wants and needs are all they care about, and the rest of the world can go to blazes if it gets in their way.
Not all are like that, of course...some of them had decent parents who taught them right....
I have seen people that were poor, had little or nothing, but whatever house they had and their kids were clean. There is no requirement for anyone to be a pig...rather it is a learned behavior...learned from their parents.
My 2 cents...
Mike