...Because Nords use weapons and shouts and Bethesda thinks you're not playing Skyrim correctly unless you're using weapons and shouts too. Bethesda nerfed magic because otherwise spellslingers wouldn't have cared about Shouts... and if you don't have to care about Shouts, what kind of Dovahkiin would that make you?
Magic was gimped so that shouts had a chance to remain relevant, but in the end they don't anyway.
Shouts are worthless anyway.
I've leveled two characters now. First was a sneak/archer on Adept. Got him to the point that I would one shot anything except ancient dragons; they took two. Second is a Nord barbarian on Master that wields a greatsword and wears only hide armor. Level 49 now and he one shots everything except draugh warlords, dragon priests and ancient dragons, which take 3, 3 and 5 power swings respectively instead. Niether of those characters have ever used any of the shouts for any tactical purpose. The only one I use regularly is whirlwind sprint for moving around town. The rest are just pointless, and their cooldowns make them even more so.
On the topic of magic: yeah, it's worthless too. I started a high elf with the idea of going heavy armor/destruction, and ran into nothing but disappointment. The biggest problem is that the perks, quite frankly, svck. For one thing, having to pick an element to specialize damage perks into is stupid. There should either be a single 20/40/60/80/100 perk at the bottom that affects all destruction spells in line with the other damage perk trees (2h, 1h, archery) or there should be a 20/40/60 perk at the bottom that applies to all spells followed by additional two-tiered perks for each elemental tree. The way it is now is inefficient, wasteful, and pointless.
Of course, all that only applies to player cast spells. That npc mage will still have no problem one-shotting you from across the room with an instant damage lightning bolt.
This could have been such a great game, but as far as I'm concerned at the moment the magic trees don't even exist. Except enchanting, of course. Because there's nothing more rewarding with trying to play a warrior style character than being forced into a magic tree to ensure that your weapons and armor are more effective than paper mache and cardboard.