This is a great question. There is an article on Kosmo Foto about the Zenit-E that I think can help answering this question. Well, two:
By Roman Yarovitsyn My name is Roman Yarovitsyn, a photojournalist from Nizhny Novgorod in Russia. I’m the editor of the photo department at the regional office of the Kommersant publishing house. …
kosmofoto.com
By Roman Yarovitsyn To understand, what exactly was photography in the USSR, you must know, that any entrepreneurial activity was strictly forbidden here. Earning money by selling photographs bypas…
kosmofoto.com
My reasoning for the Kiev-90 failure based on the above articles:
- One of the main reasons for using 6x4.5 is spending less money on film. If you check out the articles, on the USSR the photographers had unlimited access to film, tons of film, so this feature wasn't an advantage on the soviet world.
- The other advantage is lower weight and size, however I guess the soviet pro photographers didn't care too much, after all, this was the land of big, heavy cameras (i.e. Photosniper kit, Kiev-15, etc.) and lenses (mir-3V anyone??)
- Reliability. The soviet camera industry released new, more sophisticated cameras like the Almaz-103 but they weren't succesful due to lower reliability. Thus the photographers probably sticked with what was more reliable to them. In medium format, perhaps the more reliable stuff was the Kiev-60 or even the Kiev-88.
And maybe a third one:
- Gosplan: To mass-produce something, it had to be approved from above. Perhaps the government saw no reason to increase the production figures of those cameras. "Kiev-90"? "We have Kiev-90 at home, it's called Kiev-60 and use more film".