Now the architecture/os/system of the IBM S360 took some money to develop - here it says it was 5 billion $ (comparable in magnitude to the moon program - that was 25 billion $) http://www.smashcompany.com/business/cost-overruns-and-the-i... . So I guess that Kosygin was trying to avoid this expenditure.
Therefore a major point is that software development costs a lot, and they could just pirate that by copying the OS360.
On the other hand they could have done a similar thing as Amdahl/Siemens/Hitachi - copy the architecture while producing a hardware design of their own, so go figure.