The problem with ST is their manufacturing and distribution pipeline is way too slow. They could not capitalize on any of the Djoker success. Then they basically ran out of money to pay the bonuses for every slam he won.
The truth is there is more than ST just letting him go. They must have some undisclosed agreement after that. Yes, Uniqlo did sign him after that when they want a bigger star than just Nishikori. But the strange thing is Uniqlo has yet mass market the Djoker line to the world, especially after he made the USO final and then winning this year's AO. If they only want a face to go with the brand logo but never gonna sell an exclusive line, both parties would end up making less, especially for Djoker because he could get a nice percentage off every item sold with his name on it. Like RF and Rafa line.