More tanks mid than assassin

So I just caught the back half of an LCK game where there a was a mid Sion vs Gallo, lol how is it that tanks are more viable mid in proplay than assassins? And to put the icing on the cake, duskblade is getting needed on the pbe, apparently it's too strong shrugs. It FEELS like riot overreacts when assassins (in mid because kha is nicely strong) seem strong where other classes (except bruisers) are allowed to be strong for a much longer time.

Edit, I think it was a mid ryze not galio