Because he failed at what he promised to do: shrink the fedgov monster. That's all.
Look, I worked my butt off getting Reagan elected Governor and had many friends and associates working for him both then and after. His first term, getting things back in order after Pat Brown's unfortunate governorship was a success in most people's book. Things started sliding midway through his second term, and I'm sure there was a reason.
As President, Reagan could have walked the talk and taken the bold steps of dismantling the welfare state.
Why didn't he? He had broad public support even if Congress was run by the Democrats.
My own guess is he lacked the nerve after being shot by Hinckley.
Reagan at his very worst was ten times better than Nixon.
He was easily the best President in my lifetime in spite of his mistakes. We could use another one like him but I'm afraid it won't happen.
Once again, here we are slamming a solid Conservative instead of focusing on the malcontents and Leftists. Point proven by the original post.