Glyphs, can someone explain ...

Why do People recently prefer scaling Cooldown Glyphs on AP-Mid Carrys over Magic ressistence... Ive checked some Pro Builds and all of them prefer 9x scaling cooldwon glyphs over mage res. So i wanted to ask some of you skilled summoners, should i also try to go scaling cooldown glyphs or is it ok if i stay at flat mage res ... pls some explanation on this :3
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