I would think a combination of low water and high fishing pressure might be the main culprit. When the dam was raised and the lake came up over a lot of new brushy cover it was thought Roosevelt would be at least as good if not better than Lake Pleasant was for the first several years after that new dam went in, however this does not appear to be the case. What I find curious is back in the 80s and 90s I heard a lot about the big smallmouths that were in Roosevelt (the Arizona state record was caught there), but I've heard virtually nothing about smallmouths there for many years.
I never fished there all that much and did not experience great fishing there when I did. In fact, it was after getting caught there in a 145-boat tournament in 1999 that I began to consider buying property near Page and fishing Lake Powell full time. Two weeks after that Roosevelt trip my friend Dale Marenda and I went to Alamo where we got got in a 65-boat Arizona tournament (All Star Bass) and two 30-boat California tournaments. The bass boat traffic was so bad I couldn't motor from one side of the lake to other in the big part of the lake. I had to go down to where the lake narrows near the dam to cross it in safety. That was insane. A week later my wife and I went to Page, found a mobile home in Greenehaven and made an offer. The rest is history! Powell today is a much better overall fishing lake than Roosevelt ever was. While Roosevelt was probably better for largemouths and crappies, it has never had a fishery like Powell's striper and smallmouth fisheries. I'm glad we did what we did because I don't think I would have enjoyed fishing any of the other local waters like I have enjoyed fishing Powell.
Ed Gerdemann