History and reasoning behind average length of a feature film

Thats a very interesting question. Though I couldn't find a precise answer, here are some interesting takes on t he subject that I learned while researching.

From this article on wikipedia:

A feature film is a film that runs for 40 minutes or longer, according
to the Academy of Motion Picture Arts and Sciences, American Film
Institute, and British Film Institute, though the Screen Actors Guild
states that it is 80 minutes or longer.

The majority of feature films are between 90 and 210 minutes long. The
Story of the Kelly Gang was the first feature film based on length,
and was released in 1906. The first feature-length adaptation was Les
Misérables which was released in 1909.

Feature films for children are usually between 60 and 120 minutes.

Cinemablend has an interesting article on this very topic:

There's a school of thought that says 90 minutes is the perfect length
for a movie-- the length of 3 TV episodes, just enough time to get in,
tell your story, and get out without wasting any more of the
audience's time. There are countless examples that prove the rule,
economically told stories that feel perfect and tight without a second
wasted.

Finally, there is an answer on wikianswers which has a different take:

Most movie scripts are 120 pages and a page translates to roughly a
minute of film, hence most films being about just under 2 hours. It's
quite genre dependent as comedies and animations are rarely more than
90 minutes, but summer blockbusters and thrillers are usually about 2
hours.