Nexus blitz still keeps the overall changes of SR the same, while URF adds entirely new scenarios and op champs by simply increasing CDR to a very high amount.
Something that is op in URF isn’t always op in SR or blitz too, so if Riot tries to nerf a champ based on urf they will be heavily hurting that champ’s SR performance.
Riot can easily balance Blitz because comparing (and/or balancing) blitz and SR is like comparing a regular banana and an apple banana. They are both Bananas, but there are some small differences between the two that make them both kinda unique. Riot can balance both SR and blitz easily on the same patch because both game modes are basically the same.
But comparing (and balancing) URF and SR is like comparing Apples and Oranges. URF adds entirely new shit, new op champs, builds and playstyles. Riot can’t balance URF in patches that also balance the other game modes because URF and SR are almost two completely different gam modes.
Summary: SR and blitz are almost the same with very little diffences. So balance changes on either one would have very similar results on both game modes. But URF is like a completely different game mode than SR, with completely different op champs, strategies and other balance issues that Riot would try to balance.