Because US airline connections to Moscow were simply not done at the time. It is similar now; US citizens can fly to Cuba, but not on US Airways. They bounce through Canada or Mexico to get there.
Mako, I'd have guesses that the engine in question is a Rolls-Royce, perhaps a Trent 700 series popular on Airbus jets based on the spinner shape and overall configuration.
However, BOAC had merged and undergone a name change during 1974, and they were using 747's at that time. The engine pictured is quite similar to the overall shape of the Pratt&Whitney JT9D that was selected for the 747's BOAC was flying -- but the P&W engines have a much more blunt spinner snout.
There is a General Electric spinner that looks just like that, but the engine cowling is a bit short, and this engine did not enter service until after BOAC was gone.
I don't know what other sorts of jets BOAC might have been flying -- certainly the engine is a modern turbofan and would not have been on the 707s that BOAC had a bunch of.
In other words, I don't have a clue.
