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Originally Posted by dtvmcdonald

But all the big markets DID have at least 4 and in some cases more VHF assignment.

According to the sources below the VHF assignment problem killed DuMont, and very nearly killed ABC, which surived only because of a $30 cash infusion (about $600 million in today's money) in 1953 from merging with United Paramount Theaters (UPT) which allowed it to weather the ad drought until the VHF allocation freeze ended:

www.temple.edu/tempress/chapters_1400/1575_ch1.pdf
While several companies announced network plans after the war, the leaders were NBC, DuMont, CBS, and ABC. Through its policy of allocating TV stations from 1945 to 1952, the FCC ultimately determined which local and national firms would own valuable station licenses and which networks would prosper in the coming years. DuMont was not favored by the FCC. In fact, the commission’s allocation system severely hindered the DuMont network and prevented any other firm from starting a fourth network until the mid-1980s. The FCC allocated a different number of stations to each city. It then invited applications from companies that wanted to build and run these stations. Many of the first TV station operators were department stores, radio broadcasters, or newspaper publishers that were moving into television. Corporations like DuMont Laboratories that wanted to build a television network also applied for local TV station licenses. To ensure diversity of ownership, the FCC did not allow any company to own more than five station licenses. Except for these five stations per company, the FCC did not grant television stations directly to networks. As they went on the air through the late 1940s, stations filled airtime with a mixture of their own local shows and programs that were produced by the national television networks. DuMont, NBC, CBS, and ABC competed with each other to build a network of stations that would air its productions. A network’s survival depended on a strong affiliate line-up, since broadcasters had no means of distributing their programs from city to city without these affiliates. However, most cities were allocated fewer than four stations by the FCC. As a result, there were not enough stations in most markets for each of the four networks to have a “primary affiliate” that was likely to accept all of the programs that it offered. In Boston, for example, the four networks fought for time on only two stations.

jfredmacdonald.com/onutv/freeze.htm
The freeze years also allowed the networks, specifically NBC and CBS, to extend their dominance over national video. If network success lay in the ability to deliver large audiences, the talent pool and financial strength of NBC and CBS provided leverage absent at ABC and DuMont. In many markets, moreover, this leverage was magnified by the fact that TV was controlled by companies already operating NBC or CBS radio affiliates. And in small markets, where a single station might be affiliated with more than one network, NBC and CBS made wide use of coercive "option time" contracts, which gave them first rights to place their shows on the air ahead of ABC and DuMont programs offered at the same time. As Allen B. DuMont explained the situation, "the freeze reserved to two networks the almost exclusive right to broadcast in all but 12 of the 63 markets which had television service. It meant that the other two networks did not have...more than a ghost of an opportunity to get programs into the markets so necessary...[to] attract advertisers from whom revenues and profits must come.

Proof of DuMont's lamentation was in the statistics. Between 1949 and 1952 network billings for NBC and CBS rose from $9.9 million to $152.3 million, more than 84 percent of all network time sales. Figures for ABC and DuMont increased from $2.4 to $28.5 million. Were it not for a windfall of $30 million acquired through its merger with United Paramount Theaters (UPT) in 1953, ABC probably would not have survived the competition. Lacking a similar infusion of capital, however, the DuMont network continued to atrophy until it went out of business in 1955.

www.cato.org/pubs/pas/pa011.html
Property Rights In Radio Communication: The Key to the Reform of Telecommunications Regulation

The FCC's Network Inquiry demonstrated how the rigidity of allocation and assignment criteria limited competition to three major networks. A strong national network would require access to the top 50 markets, where most of the viewers, and therefore advertising revenues, were located. Under the FCC's 1952 TV allocation and assignment scheme, only seven of the top 50 markets received four or more VHF assignments. Twenty received 3 VHF assignments, 16 received two, and 2 markets received only 1. "As a consequence of this scheme, one network could reach 45 of the top fifty markets with VHF stations and the second could reach 43, while a third network could reach 27 and a fourth would have access to VHF stations in only 7 of the top fifty markets." The same FCC report documented how the DuMont network crumbled in the '50s as a consequence. FCC Network Inquiry Special Staff, "The Historical Evolution of the Commercial Network Broadcast System," October 1979, pp. 77-79.