Originally Posted by Rebonack
Blue buff gives tremendous MP/5 in addition to a significant amount of CDR.
Between Runes, Masteries, Blue Elixer, and Blue Buff a Mage on summoner's rift has all the MP/5 and Cooldown Reduction they'll ever need. This allows them to focus purely on amping their AP as much as possible. And the longer time between team fights means that they have their Ult ready on a more reliable basis.
On top of that Mages unusually have a level advantage on soft targets of choice (Support and Rangers) which likewise makes it easier for them to do their job. None of these advantage are available on the Crystal Scar and thus Mages need items that will make up for it.
riot designs the game, players often find a way to break it, i mean not even rioters are really familiar with the DOminion meta and even them took ad mids on those times before players started taking AP to mid, mages were designed with the blue buff in mind which holds even more true for Anivia but most of them were balanced around the idea of having mp5 runes hence why on the earlier champ spotlights phreak takes them on many mages instead of the now removed dodge runes.
you can aslo take the same logic to champions, see SHyvana malphite, Mundo and ezreal for example
Malphite: Malphite was the first tanky guy i "fell in love" with when i started playing, it was also a malphite free week but every time i tried to pick malph everyone raged at me dodged me or told me to pick another thing because malphite was considered really Up at that moment (that was the past july when Leona was new), he didn't receive any buffs whatsoever and i didn't unlock malph because everyone told me to save my ip for better things, suddenly a high elo discovers how to rock with malphite and he's deemed op and nerfed (DOminion community also has her hands on it but i bet it was mostly becaus eof the high elo and his participation on the lates tourneys)
when she came out everyone said, yeah cool concept fast clear time but you know? she has rage problems and her ganks sux ass, she can't even kill anyone, however some guy was "stubborn" enough to keep jungling with her until he discovered a reliable skill order, build and way to gank which leaded to one of the best junglers, same cas applies ffor Mundo, i had never seen a jungling mundo, or a Mundo at all until this year when they discovered mundo's power and then Mundo got nerfed.
Ezreal: SOme time ago a high elo player questioned the viability and strenght of Ezreal and made the playerbase rage and cry for Ez buffs, they got what they wanted on a small fashion Riot reducin Mystic shot's mana cost by 10 at all Ranks and reduced the cooldown on rue shot Barrage, they released pfe giving former ezreal mains a reason to play Ez againm and loyal ezreal mains a reason to continue playing Ez, lately those guys that are actually good with ez have been Facestomping on every mode but the treeline and the forums are rampaging for Ezreal nerfs, more than the typical hundred of threads asking to nerf Darius daily, and what are the sr players doing is simply not rushing triforce nor sheen