Because the club have tried there best to airbrush the entire thing out of the history of the club, they lost all right to moan about the abuse they get over heysel the moment they brought the steau banner out and there nice good fans who never do anything wrong creamed their keks over the great bantz basically to a last man of them.
You try score points over a tradgedy your clubs fans caused, then expect to get the abuse you deserve for it, any one of them who liked that banner for example or who brings up any excuses like Chelsea fans, torino ones, or juve started it, the police's fault, or that times where different back then deserves those chants thoroughly.
And it hurts them to hear it cos it reminds them of something they want everyone to forget about.
One only has to read the Heysel "commemoration" plaque stuck somewhere on a wall outside The Pit.
It simply refers to 39 fans dying.
A visitor from Mars would be forgiven for thinking the tragedy befell fans of both clubs.