The only reason Thunderlord's is so popular is because what else is an AP champion going to get?
Ferocity:
Warlord's? Absolutely not.
Fervor? Absolutely not.
Deathfire Touch? Still garbage on all but a few AP champions
Cunning:
Stormraider's? I could see occasional cases where it's useful, but many mages don't need movespeed AFTER bursting an enemy (when their cooldowns are down). This would be my second choice on most mages, though.
Windspeaker's? What mage (aside from maybe Karma) has a shield they use on allies?
Resolve:
Grasp of the Undying: Why? A mage who is in range to auto-attack is dead.
Strength of the Ages: Generally seen as garbage even on tanks.
Bond of Stone: For a solo-laner, this will not be useful most of the game. Also what mage wants to take damage for a teammate?