Because every character that functioned like Ryu has been nerfed heavily in their overall punish game. Biggest example is Luigi. Another example (but less relevant) is Bayo (this is debatable because many of her combo starters did not exactly suffer from range issues). Ryu should not be taking stocks at below 100% from one or two mistakes, even without rage. Also I won't make this into a whole text wall of arguments, but basically my reasoning is the same as Trifroze's argument about why heavy characters like DK and bowser should not have kill confirms that kill extremely early. Although for heavy characters their frame data and lack of a projectile somewhat offsets their ridiculous kill confirms, Ryu has a projectile, super OP up-b and plenty of movement mixups from his down-b, along with the best frame data in the whole game (those tilts and aerials are disgusting). Also beacuse of Ryu's frame data (and easy-access shield-breaking moves from his tilts), he can reliable pressure shields all day so avoiding his tilts for the duration of a whole set is impossible. Furthermore Ryu's kill confirms work for a wide range of %s unlike DK and Bowser's kill confirms.
I have seen all of the pro-Ryu arguments, and they sound awfully similar to what me and other Luigi mains have said regarding Luigi's own punish game. Regardless Luigi's punish game was patched out, and similarly Ryu should have his punish game patched out should rely on reads for his kills instead of dumb "insert light tilt/jab" > true shoryu or dair.
The only other arguments from pro-Ryu people have been that his inputs are hard to do, but the best Trela has stated that this really isn't the case (look for his "inspiring" post a few pages back). Also unless Ryu's technical barrier is in a similar level to melee multi-shines and wave-shine confirms, I do not see this "technical barrier" argument holding ground. Furthermore pro-Ryu people state that Ryu's aerial mobility limits his ability to use his punish game, but this did not stop Luigi from getting nerfed as well.
Hope I have made my case clear. BTW I do not explicitly think Ryu as a character is OP but that his confirms are and should be toned down or removed so that he requires smart mindgames from Focus attack and smashes for kills, just like most of the other characters in this game's roster.
Dre89
The issue with Diddy is not the fact that he kills early (although he still can kill light fastfallers very early with a bit of rage and staled d-throw/u-throw into up-air), but the fact that he has a consistent kill setup that is hard to DI properly due to his mobility and how quick d-tilt comes out (frame 4) Also at higher % he has a stock cap kill throw and aerials like fair or bair to confirm from d-tilt (bair in particularly scales very well with rage due to its KB growth). This is also compounded by his abusive neutral game and his banana, which wrecks low-mid and sometimes high level players. Kill confirms that work for a variety of % range have all been toned down or removed from the game, so considering Diddy's neutral and his overall very powerful and versatile moveset diddy really should not have the kill confirms that he does atm.