If I think about it I probably can pinpoint the reason for the soldered cable looms: not only in SSSR but across the whole eastern block in general there simply weren't any reasonably cheap and reliable board to board or board to cable connectors for general use with reasonable density.

Edit to add: there were various ribbon cables used in eastern block electronics, but mostly with hand soldered connectors, not IDC. Two exceptions I can think of are Shugart-compatible floppy drives (although some Czech computers actually use ribbon cables with hand soldered board-to-card connectors) and some Metra-branded measurement equipment which uses MicroD-like IDCs internally.