There have been many failed gold medalist at HW. Besides that, any time Harisson faced anyone that wasn't a bum he lost. He hasn't beat a C level fighter in his entire career and he gets knocked out embarrassingly by B level fighters. It's funny that anyone considers him a threat or a significant win for any good fighter.
Because of British media propaganda and the casual fans (and some hardcores) frightening gullibility.
The honest answer is he won a gold medal so became very famous household name, then he became a villain, so people like to see him beaten up. And then they keep sticking him in prizefighter because he's a famous name and he keeps winning it.
Because he is an inspiration. He never give up on his dreams. If Quitali had half the heart A Force did.. he wouldn't have quit against Byrd.