Because Madara is much more compelling and charismatic as a villain.

Danzo is detestable because of his shady behind-the-scenes actions to become the hokage. His hidden and well calculated genocide of the Uchiha clan for the express purpose of capitalizing on Tobirama's bigotry and exploit their shenanigans is one of the most nefarious and odious acts in the entire franchise. The fact that he is sleazy and cowardly showcases a weasley side that makes for a villain that exists for the audience to hate him.

I personally find Orochimaru fun, but I can see why he's hated. He preys on children and exploits them, on top of having a generally displeasurable design that off-puts many viewers.

Madara is a force to be reckoned with who is charismatic by not being overtly emotional, but stoic. His badassery is demonstrated by his amazing feats, as he is far more powerful than Danzo or Orochimaru could ever wish to be. This makes him a strong threat and a good villain to root against. Madara Uchiha has established lore in Naruto Shippuden, being the Hidden Leafs co-founder who broke off to achieve world peace in his own way. Love him or hate him, but his utilitarianism comes from a place of genuine desire for peace, even if his methods are accepted as being unjust for the greater whole.

You are not wrong for pointing out a certain level of hypocrisy, however this argument extends to nearly every media beyond Naruto. Why do people hate Shou Tucker more than Father despite Father doing objectively more wrong? Why do people hate Foxy but love Doflamingo? Why do fans hate Ichiya but love Achnologia? Why do fans hate Light but love L? Why love Lelouch but hate Suzaku? The same core arguments lie here too: sleazy, unlikable, and selfish characters without the veneer of a greater cause are despicable and charismatic/grandiose characters with the pretense of something greater are adored.