Not only that, Mr. T, but when Monty Python made this film, it was their first one ever. None of the productions companies would endorse them because they thought the movie would be a flop (HA! Now there's one for the record books!). All in all, the 6 Pythons only had a budget of about 100,000 dollars to make this film, which, when you count hotels, car rentals, food, salary, etc. is not a lot of money. Therefore, they probably didn't have the chance to hire a proper crew or anything. I mean, come on, in terms of seeing the microphone, there have been a lot bigger screw ups than this one, especially in the most recent movies when this happens, and you have the option of computer graphics.
You're right about the small budget there. They were initially going to use real horses, but they turned out to be too expensive, so they resorted to coconuts instead. It's fantastic how they managed to use them as an important part of the plot, though. "What... is the average airspeed velocity of an unladen swallow?" "African or European?"