Because with smithing you still get a decent ammount of experince at higher levels, other skills are really grindy at higher levels, that and since you get the same exp for smithing anything smithing can be cheap and fast to level up. Going specifically on enchanting, you have to buy the soul gems, fill the soul gems, get some armor/weapons to enchant, go through each of the menus for every item you want to enchant, and repeat. It's very time consuming and rather expensive. Then you have the fact that there seems to be no master level enchanting trainer in skyrim. so from 75 to 100 all that training has to be you at an enchanting table. There is a master enchanter in Markarth. I personally collected all my soul gems from creatures. You can find many of them in Dwemer ruins or necromancer dens.