It is my understanding that the category "Mysticism" was dropped, and some of the skills in that category were moved to other magic categories.
From the Todd Howard http://theelderscrollsskyrim.com/bethesdas-todd-howard-podcast-interview/ at the 16:12 mark.
"So people see we’ve removed Mysticism, but that’s just a label right? Those spells go into other skills. And then it gets deeper within those skills."
Edit: Add - Most likely ninja'd lol. However, if there is a lore reason, I don't know that it has yet been stated.
Well thats Oblivion Mysticism spell effects
Detect Life, Soul Trap, Dispel, Spell Absorption, Reflect Damage, Telekinesis, Reflect Spell
Thats Morrowind Mysticism spell effects
Absorb Attribute, Absorb Fatigue, Absorb Health, Absorb Magicka, Absorb Skill, Demoralize Humanoid, Mark, Recall, Detect Animal, Detect Enchantment, Reflect, Detect Key, Soultrap, Dispel, Spell Absorption, Almsivi Intervention, Divine Intervention, Telekinesis
Thats Daggerfall Mysticism spells
Banish Daedra, Detect, Far Silence, Fenrik's Door Jam, Holy Touch, Holy Word, Null Magicka, Open, Recall, Silence, Soul Trap, Spell Reflection, Tongues, Wizard Lock, Wizard Rend
So number of spell effects was reduced in Oblivion some of them even without any reason like Mark & Recall, Divine Intervention, as well as most of them was moved to another schools also why hostile absorbs become part of Restoration School?
Only reflect damage was added as new feature in Oblivion.
And after so much simplifications Todd say Mysticism was useless?
Oh well devs by self make it useless and now even more delete this school and move rest of spell effects to other schools, what spell effect left in game we don't even know.