I could see that being it. In fact that frame let them get away with a piece of simple flat glass in the front. But you would think that somewhere along the line they would have gotten the production of bubble glass down pat. According to the article these planes were produced into the 1980s. But maybe later in the game it just wasn’t a priority.
they did make bubble canopies but for later jets. they would not retrofit mig 21s that were now outdated and 2nd line fighters.