"Because he's the hero Gotham deserves, but not the one it needs right now"
Because the hero Gotham needed was Harvey Dent. Gotham needed hope, which Harvey represented. If Gotham knew that Harvey went evil, then it would have shown that the Joker was right. It would have shaken peoples faith and motivation to do good.
It's because the movies are written by christopher nolan, and that guy does not have good politics. The other guy is right with their explanation, but the underlying message is, as you say, pretty much total nonsense.
And to avoid that, all they have to do is became big damn heroes by giving their money in charity, or tax, or fund a research lab or whatever way of throwing their money back out there that they choose.
Astounding that they'd find it so detestable that they'd rather risk death in the hands of a class revolution than see their money feed kids or cure cancer or whatnot.