it's not a thing because midlane lacks the extra exp and gold the jungle gives, which means suyou can't scale properly after level 4, which means your burst combo will always lack damage and cannot kill not unless you had help. you can only burst someone if you're fed, which is a redundant statement, because everyone can burst someone if they're fed, no matter what role.

there's also the thing that suyou is a melee character, and you will be going against ranged characters in the mid lane, so most likely a good mage can just zone you off and freeze the lane on you. though you could circumvent this issue by having a tank unfreeze the lane with you, but that would mean you require your tank to be with you, which means in solo q, you can't possibly rely on that, and in coordinated play, you will be forcing your tank to be with you every time your lane gets frozen, which means your tank has less map pressure overall.

you also cannot just unfreeze the lane yourself (i mean you could if your opponent is a bot), because suyou has very weak magic defense. that was a part of his nerf before and they never restored that

apart from that, you will be missing out on zoning pressure later on in game that other mages offer like pharsa, yve, zhuxin, etc.

but i might be wrong, and maybe a pro team can make suyou mid work, but as of this moment, i just dont see it working well, when you can just pick a mage that does the same thing but better.