>more like a theatre production than a feature-length film
Because it was basically a "television play", a hybrid between a theater production and a higher budget TV series/film. Such things were common on Soviet TV.
One has to understand that Soviet Union was relatively poor when compared with the west, especially so during perestroika.
For example "Ali Baba and the Forty Thieves" is another Soviet tv play https://www.youtube.com/watch?v=pCutJ-7acvg