Revealing mistake: When the wall is falling you can see the wires pulling it over.
Continuity mistake: When David retrieves the rocket and the great fear chases him through the town, the sun goes from setting to being right above them.
Factual error: One of the male character is shown wearing a BDU, or Battle Dress Uniform, which wouldn't be developed until the late 70s and wouldn't be released until around 1982, 10 years after the storyline.
Answer: Likely for a variety of reasons but probably mostly for cost reasons. It is cheaper to film movies in Canada due to the currency exchange rate.
raywest