Because if you know how to use his Q right, you can score in free harass and farm. He has a 4.0 AP ratio channeling heal that also provides a tremendous amount of Armor and MR, free AD (helps early game and 70 AD is massive later on, even if you're AP), and an ultimate that can potentially reset his entire kit if used correctly.
I want everyone to start using him just so Riot can finally nerf his ratios to the ground and everyone will lose interest after. He isn't that difficult to lane against or shut down in a teamfight, but it's his potential to snowball after effortlessly securing a few kills and assists with his Q and R that really screws up the other team if they're behind.
"This will really separate the good Master Yi's from the great Master Yi's." --- The statement below is false. The statement above is true.
He is getting attention because bad people that don't think his kit through think he is amazing because he has a tendency to get high killchains, which he did in a big tournament. The thing is that he is the very definition of feast or famine. If his team can set him up a kill or two then he can quickly turn it in to a quadra or penta. If his team can't effectively do that work for him, they end up with a short range AP caster with a single long cooldown nuke.
M5's Master Yi excelled because they are one of the strongest teams playing the game and they can easily outplay many of their competitors. A Master Yi with a much stronger team behind him than yours is going to obliterate you.
He isn't terrible in solo queue but he is obnoxious because he is basically asking his teammates to do all the work so he can hammer Q and take all the kills, otherwise he's largely worthless compared to the enemy's AP caster. --- "Happiness is nature's way of telling human resources you're overpaid." - Catbert