I wanna say that its because his designition as a split pusher allow him win game if his teams aren't completely retarded and abuse his ability to essentially turn into a "Choose how you lose" Lane
Also probably because he's a very marketable kit. Your Q helps you last hit while also providing sustain, and rewarding those who last hit with it more, Your E allows you a slow while also doing damage, And letting you boost towards your enemy. Your W allows you to trap people for a short amount of time if you can use it in a sensible time scale, And your Ult essentially turns a 1v1 into a 2v1, or even a 1v2 into a reasonable 2v2
However, My big problem is the "DPS" Part, As yorick isn't made for DPS, But a more bruiser-esque style, and such, by dedicating more gold for damage, You're losing out on his ability to become a reasonable threat due to the fact that you already have 2-6 potential sources of damage at any given time, And once you die, Your summons start dwindling faster than your chances of winning.