Because everyone needed a gap-closer on top of flash...

Protobelt will cover a lot of champion's specific weakness in not having any gap-closing abilities. Champions like: This will make them broken (in a scene) because their kit is balanced around not having any gap-closers. Not to mention champions who already have good mobility purchasing this and getting even more. While flash still is further distance and overall better, this still seems a bit strong to be on an item that also gives CDR and 60 AP. While you cant dash over terrain, you certainly can dash into brush or basically negate a slow like from a Teemo mushroom not to mention will never hit all the shots anymore since you can dash at least one.

BTW this well be op in URF (like most things) and possibly very op in ARAM (snowball + flash + protobelt = all the engage and disengage you need)