It's because, before 1925, to record sound, you had to stand in front of a big horn, like this one. If you wanted to record both audio and video at the same time, your scene better involve a big horn, because it was going to be featured prominently in the shot.

In 1925, Western Electric invented the first practical and mass-produced electric microphone, the kind that news reporters use. That made it possible to actually film a scene and hide the microphone from the camera.

A couple other issues that needed to be overcome were how to reliably synchronize the audio and video, and also how to amplify the audio to be loud enough to fill a theater. Before that, a record player at home would have a big horn to project the sound. That's OK if you're sitting next to the record player in your living room, but it doesn't work if you're sitting ten rows deep in a movie theater.

In 1924 and 1925, General Electric and Bell Labs developed competing systems of "moving coil" electric speakers. This made amplification possible, and allowed theaters to be fitted with speaker systems that could be heard by all the patrons.

Still remaining was the issue of reliable synchronization. There were two competing sync systems in early sound film: Vitaphone's "sound-on-disc" system, acquired by Warner Brothers and used on their first sound feature films Don Juan (1926) and The Jazz Singer (1927). It used mechanical equipment to keep the discs and film running at the same speed, but it required the discs to be replaced after only a handful of viewings.

Fox Film's Movietone system was a "sound-on-film" system, that printed the sound from electric recording equipment directly onto the film itself. The technology was first used in Fox's newsreels in 1928, and within a couple years, it took over as the standard.