Voyager was unlucky to be aired only on UPN, a network that existed for just about ten years and never really managed to gain much of a toehold in America. In most markets, UPN refused to allow independent stations air Voyager unless they agreed to become affiliates--even though there was never much to offer except a few evening programs, like Voyager. Here is a quote from wikipedia:
Although it was considered a major network by the Neilson ratings, UPN was not available in all areas of the United States. In some areas, UPN programming was shown off-pattern by affiliates of other networks or by otherwise independent stations, such as in the case of KIKU-TV in Honolulu, Hawaii. Some affiliates were also known to extensively preempt network programming in order to broadcast local sporting events.
At its demise, UPN supposedly had about 200 affiliates, but that is misleading because many of the stations were not even carried on cable.
It's difficult to compare the ratings of TNG, DS9, and Voy because the first two were syndicated and Voy stuck with being a network show. I personally think that Voyager would have been much more popular if it had been marketed the way TNG and DS9 were, but we'll never know.
