I don't know.
Maybe it is because 4E introduced At Will Wizard powers that could do nearly the same damage as At Will powers from other classes, but then went back to wimpy cantrips.
Maybe it's because the Shield spell could be stretched over multiple encounters in 1E to 3E, but in 5E lasts a round.
Maybe it's because multiple protection spells could be cast in a single encounter to protect fellow PCs, but with concentration, now it is one spell.
Maybe it's because NPCs now get a saving throw every single round.
Are you actually being objective? I cannot be the only one who sees how weak saving every round is, or one protection spell at a time is.
Wizards could often have up multiple spells at the same time, not just protection. Now, a flying wizard has virtually zero protection up (except Mage Armor and possibly Mirror Image). A wizard can no longer be both invisible and flying (unless someone else casts one of the spells). There are entire areas of tactics and strategies that are not only nerfed, they just no longer exist. Protection of others is basically just thrown out the window.
I actually like many of the higher level nerfs, but the lower level nerfs are just unnecessary.
Fog Cloud in 3E has a 20% miss chance at melee range and 50% miss chance at longer range. Now, the chance to hit at any range is the same as if the spell wasn't there (advantage cancels out disadvantage) unless nobody is actually in the Fog Cloud (in which case it's disadvantage). One cannot be protected within the spell radius anymore (shy of the DM ruling that the PC can move around and the NPCs cannot hear him and do not know where he is, that's DM dependent).