Allen DuMont was a early pioneer in TV broadcasting. He had one of the first TV stations and manufactured TV sets and also equipment for TV stations. In fact, he was probably second only to RCA in suppling TV stations with cameras and video equipment. The first TV station I worked for in 1964 still had some old Dumont monitors in the back room.
His problem with the network was that unlike NBC, CBS and ABC it was not associated with a established radio network- this gave the those networks a big advantage in signing up affiliates because most early TV stations were built by established radio stations. The associated radio networks also gave them a large talent pool and the resources for programs- since most early TV programs were adapted from existing radio programs.
The DuMont network had none of this and most of what they showed was sports- especially boxing, but there just wasn't enough affiliates to make the network profitable.
By the way there was another fourth network briefly in 1970- UBC- the United Broadcasting Co. It picked up a few UHF stations and lasted about six months.