Why AP champions cant have spellvamp but AD champions can ?
Riot removed WoTA but kept BT,Death's Dance etc. Also they removed spellvamp from Ryze with the rework.Lets think about it.
Ryze(Stormraider's Surge,RoA,Archangel's Staff,Void Staff,8450 gold) against Yasuo(Warlord's Bloodlust,IE,Statikk,Hexdrinker,Vampiric Specter,8400 gold);
Ryze left the trade with %30 hp and Yasuo with %10 hp.
Obviously Yasuo will get to %100-90 HP with 1 wave.
Pre-rework Ryze could get to something like %70-80 HP with 1 wave.
Current Ryze cant do anything about sustain and must sacrifice a tower.
Remember ! The main issue is sustain.Not Ryze or Yasuo.
Also sorry for my simple english.
offers what's essentially spell-vamp: but better. Things like Cho'Gath's Vorpal Spines, Akali's Twin Disciple, and Teemo's Toxic Shot actually benefit from Gunblade's Vampirisim where spell-vamp as a stat never aided on-hit effects.
allows burst-damage champions a burst of survivability to burst, gain a burst of defense, and then burst again.

and
already give them "spellvamp" on physical damage spells. Which, again, begs the question, what's the difference?
? and if you pair that with
would increase the heal you get by 25%?
which allow your next basic attack after a spell casted to do bonus ad damage and ap damage and if you have lifesteal and spell vamp from the vampirism mastery it would allow you to get 2% from each.