It's difficult to boil down their deal to a single sentence but the general gist is the technology was available and they seemed like a good idea at the time. I answered a similar question here and there's a couple of things to stress.

First, despite what it may seem like from the student or public side of the teacher's desk, schools and teachers have been fairly progressive with regards to technology in the classroom. Following World War II, the US Army donated thousands of cameras and projectors to American schools (as I explain in the older response, they weren't equally distributed across the country and more likely to be found in mostly White suburban or urban schools), which made showing the films in school fairly easily.

Second, by the 1940s, there was a fair amount of research related to the power of images in helping students learn content. The general sentiment was that films were more interesting to students than filmstrips or pictures in a textbook, so if a topic could be taught with a corresponding movie, someone would be willing to use that film. Michelle Anne Boule, in her dissertation Hot Rods, Shy Guys, and Sex Kittens: Social Guidance Films and the American High School, 1947-1957, looked at the themes of many of those films and made connections between social norms and post World War II America. That is, there was a feeling from adults the next generation of young people didn't know how to sufficiently "adult" as the war had some completely disrupted their adolescence. This sentiment also extended to new factory workers, etc. Those in charge (i.e. older White men) felt certain things needed to explicitly laid out. An entire industry around short education film-making emerged in the 1940s and 50s.

And that all came crashing to a halt following Sputnik in 1957. It's unfair to trace all changes related to education and educational films to a single event but it is fair to say the launch of the Russian satellite was the first domino. Almost overnight, the focus of education narrowed to math and sciences, which meant a whole bunch of content and supporting resources were jettisoned in schools across the country in an effort to "get back to basics." Films continued to be used, and then filmstrip, and laserdiscs and then VCRs and DVDs, etc, but the market mostly dried up for short educational films.

Finally, as I mentioned in the older post, there's always more than can be said about the use of short films in the military and industry!