There were a number of reasons, but here are the most important:

a) NBC, CBS and ABC had profitable radio networks which supplied cash to their TV divisions during the early, unprofitable start up days of TV. DuMont did not.

b) Most cities only had three channels allocated by the FCC. Some only two. These were grabbed up by NBC, CBS and ABC. DuMont often could not get full time station affiliations except in only the biggest cities (NY, LA, Chicago). These cities had more channels allocated so there was room for a fourth.

c) TV stations were often started up by established radio stations (WBZ, WBAL, WWJ, etc). These stations were often NBC, CBS or ABC radio affiliates and those networks insisted that the TV station carry the radio affiliation. If you refused, they would pull the radio affiliation.

There were other reasons, too, but these were some of the major ones.

The DuMont TV Network (1945-1955) had a more urban look, since they were primarily seen in big cities. They also didn't have the radio stars (Jack Benny, Bob Hope, etc) to transfer to TV (they were locked up with NBC, CBS and, to a lesser extent, ABC) so DuMont had to create their own stars. Most prominent of which was Jackie Gleason. But when Gleason made a splash in the NY ratings on DuMont, CBS hired him away with a bigger deal and more stations.

After the network folded in 1955, the DuMont owned local stations (WABD, New York and WTTG, Washington) were sold and became the nucleus of Metromedia, which in turn, was sold to Rupert Murdoch and is today's Fox Network. WABD became WNEW-TV and is now Fox 5, WNYW. WTTG's call letters were never changed but it, too, is now a Fox owned station.

The ulta-modern (in its day) DuMont TeleCenter on East 67th Street in New York (opened in 1954 as DuMont's last hurrah) is now the Fox Television Center.