Because Diddy Kong has almost just as good of a neutral game as Sheik does and outplays the Sheiks. Diddy F-Air outranges Sheik's Fair. Dtilt into grab is a true combo at most percents on Sheik. Banana is just so good. People underrate Diddy so much now just because people are overlooking ZeRo because "Oh its just ZeRo he will win anyways" but when Ranai wins with Villager a character with less results and less reps they say "Oh Villager is top 5 because of Ranai".
Yeah, Diddy is still an immensely solid character.
When they nerfed him and then later buff his smash attacks, what they really did was refocus his game around banana play, which is how it was supposed to be from the start.
Banana play has counter play, at least (ie get the banana away from Diddy and force him to guess as to when you're going to hit him with it).
I feel like this post myopically underrates how relevant Sonic, Rosalina, Villager and possibly Pikachu would become in the absence of those two. Super-heavyweights can absolutely not exist while either Sonic or Villager is in the game.
Sonic and Villager are generally bad matchups for HWs, but nowhere near as bad as Sheik/ZSS.
For Bowser, villager is all about powershielding and patience. It's seriously about picking the right times to get in and get some damage and it's actually okay to stay away from villager the remainder of the time. Bowser can also
pop both of villagers balloons without hitting villager.
Sonic is a different story, but Sonic actually has to get IN on you to really do any damage, and while he has the tools to keep you guessing, pivot grab shuts him down hard and he at least has to take a risk to damage you.
Sheik and ZSS don't have to risk ANYTHING to completely demolish all of the HWs. Sheik can needle camp them forever and they can never catch her (I think DK actually has the best Sheik matchup of the bunch and even his is pretty bad) and ZSS can poke with paralyzer to confirm into the grab or just SH nair HWs in the face all day because the only way HWs can punish that is by powershielding it and then using like a frame 7 or better option, which most of them don't have (Bowser has it in OoS fortress and maybe Charizard's jab?). Her grab can also catch landings VERY well, especially on big targets.
Pikachu is probably a more problematic matchup for HWs than Sonic or villager, but even he still isn't as bad as Sheik/ZSS because, again, he needs to get close to do any real damage and his projectile is easily powershielded. His quick attack sucks to deal with, but it's possible to punish it unless he does it perfectly every single time.
Rosa is also problematic, but again, she at least has counterplay in removing Luma from the picture. She could probably stand to have some of her more ridiculous hitboxes shrunk a bit, and maybe focus more of her killing power into Luma (really, when Luma is dead, Rosa should almost never be able to kill with a smash attack).
Really, the trouble with Sheik and ZSS for HWs is that they can do pretty much everything they need to do in relative safety. They don't need to risk anything while any misstep by the HW can often mean losing a stock or at least getting comboed for 30-50%.