What were the factors that lead to this length?
You may want to check out this article. The author goes over statistical info for all the theatrically released movies on IMDB.
And this may surprise you, but on average movies aren't 2 hours long. They are generally 90 minutes or so long. And they have been about that long since the 1950s or so. In recent years movies (on average) are actually getting a few minutes shorter.
However the most popular 25 movies of each year are consistently longer than the average movie. They started a sharp climb from about 1930, started crossing the 2 hour mark around the mid 1950s, then declined in length till the 1980s. It wasn't until the late 1990s that the average blockbuster was consistently over 2 hours long.
hi! fyi, there have been a couple of previous threads on this topic; check them out: