Can I ask why? Genuinely curious here

Slytherin students didn’t have better academic performance and they certainly didn’t have better behavior than the other houses. What they did have was a head of house who would award his own students points for almost no reason while handing out penalties to other houses like candy. If Draco Malfoy answered a question correctly in potions, he’d be awarded ten points, while Hermione giving the same answer would lose ten points for being a know-it-all. 

That’s the thing, the game was rigged in Slytherin’s favor. Snape set his own house up to win, through absolutely no merit of their own, seven years in a row with no penalty. Meanwhile Dumbledore is made out to be the one who “just hands victory to his own house” after four members of his house put their lives on the line to save the school from a genocidal mass-murderer. 

Gryffindor deserved the house cup because their students saved the school, but even if they didn’t, Slytherin should have had it taken away from them because they didn’t earn it. 

I can’t even condemn Dumbledore for letting Slytherin believe they’d won, sit in a green-and-silver dining hall, and then changing it when he announced they’d actually lost, because after seven years of cheating, it’s not enough for them to just lose. If they’d just lost, they’d think they were cheated out of something that’s rightfully theirs. Allowing them to believe they’d just once again been handed an award they didn’t deserve, and then giving it directly to the house that actually did something to deserve it, teaches a valuable lesson. 

Anyway, if we’re going to criticize Dumbledore’s abilities as a school administrator for anything, it’s how unchecked he left Snape’s treatment of his students. Even putting aside the emotional and physical abuse he inflicted on his students, there should have been some provision in place to prevent his abuse of the points system before he had a chance to hand it to his own students for ONE year, let alone seven. 

There should have been a provision that the current holder of the house cup is ineligible for participation in the next year’s competition. There should be an upper limit on how many points you can take away from another house’s students, and how many points you can give to your own students. Students should be able to appeal unfair penalties to the headmaster. 

Point is, Slytherin shouldn’t get an award just because their head-of-house refuses to play fair

And to people who say “but that reinforces the anti-Slytherin perception of the school” - here’s the thing, it doesn’t. What people are quick to forget is that Gryffindor was actually tied for the Cup until the Norbert incident, and when that happened THE WHOLE SCHOOL was shitty at Harry, Hermione and Neville for screwing up the chance at a non-Slytherin victory for the first time in eight years solid. Slytherins were literally going up to them and thanking them for making sure they didn’t have to actually work for their victory, and the other three Houses were giving them the cold shoulder. The whole school was ALREADY tired of Slytherin being handed the win - and please don’t try and tell me Slytherin earned every single win or that Snape was just trying to level some imaginary inherently anti-Slytherin playing field, I will laugh at you.

I see even people who think Gryffindor earned the win lament the feelings of the Slytherins, “OMG the Slytherins rilly rilly rilly thought they’d won again, and that mean old Dumbledore let them think that just so he could have a dramatic win for his precious Gryffindors! Why didn’t he award the points when Harry was still in the hospital wing? That would have been fair! Imagine how they cried that night, how much humiliation they felt! They were so happy and Dumbledore let that happen just to take it away, how could he do that to children?!” What every single one of these arguments manages to miss is that Gryffindor, Hufflepuff and Ravenclaw students are also children. Are they not? Or are they short adults, obligated to take every loss maturely with a smile and an ‘oh well, guess we just need to try harder next time!’? No. They’re children, and they’re not happy, and why should they be? Three Gryffindor students literally risked their lives to defeat the greatest evil Slytherin House had ever given to the world, and that still wasn’t enough. Slytherin was going to walk away with the win yet again for the eighth time in a row. That’s it, it’s official, there’s no point in trying, bye bye. And then, somehow, miraculously, things are set right. Bravery and cleverness and loyalty and all the things those three Houses most prize, are given their just reward and celebration.

With that one act, Dumbledore told every muggleborn and halfblood in the school “your rights are worth defending, your muggle heritage is not a crime that disqualifies you from being here, and preventing the person who wanted you exterminated from coming back is worth all the reward in the world.” But its the feelings of the Slytherins, the ones who had literally gotten complacent and assumed the Cup was theirs by right, the ones who literally resented having to work for said Cup, that this fandom gives a damn about because they didn’t get what was essentially a shiny toy. Honestly, I don’t trust Snape not to have tried to skew the results back in Slytherin’s favour if Dumbledore had given the points earlier. Apparently, neither did Dumbledore.

It was Dumbledore letting Snape get away with stacking the deck that reinforced anti-Slytherin feeling in the other three Houses, not him saying ‘no, actually, these people are getting a public reward because they deserve it’. Dumbledore showed three quarters of his students that yes, it’s worth trying, worth doing the right thing and taking on the impossible even if it seems like the deck is stacked against you. And he showed the final quarter of his students to not get complacent and assume people in authority will always grease the hinges for you. A whole year of students - Tonks’ year, in fact - went through Hogwarts with Slytherin winning every single House Cup. The other three Houses never once knew the joy and pride of winning - and here’s the thing, the Slytherins never learned what it is to lose, which is an absolutely vital thing to learn. Snape (and Dumbledore, by failing to intervene earlier) didn’t do that year’s Slytherins any favours.