Its extremely simple really, Ubisoft wanted people to buy it directly from them or places where they get a greater cut of the profits, then released it a while later on Steam where they could mop up all the people waiting for it on that platform. Can't blame them.|
They probably think that Steam with its huge discount sales devalues their product and to be fair, it probably does.
Steam - iN5OMANiAC
PSN - iN5OMANiCAL