Well, it's the money, of course. Launching a new TV network is expensive. How much money? Well, we're talking into the hundreds of millions just to get some decent level of distribution.
First, let's consider the possibility of launching a new cable network, as Fox News did back in the mid-nineties.
The strategy that Fox News used at launch was to give cable companies a sizeable chunk of money for each subscriber that they offered Fox News to. My vague memory is that the launch incentive was somewhere around $10 to $20 per subscriber. So for 20 million subscribers (which is less than 20% of the TV households in the US), at $20 each, that would add up to $400 million.
And it still probably wouldn't work. My strong suspicion is that an independently launched progressive TV network would not get significant carriage even with cash incentives. Why? Because independently owned cable networks (that aren't part of one of the large, existing media groups) struggle to gain carriage, regardless of political content. There hasn't been a successful launch of an independently owned cable network in well over ten years now...
So the alternative is to look at over-the-air television stations. Buy a full powered station and you can invoke "must carry" to get on cable and satellite services for the market the station is in. A couple of failing TV stations in the Dallas-Fort Worth area recently sold for about $10 million each through bankruptcy court. One in Houston sold for $15 million. NBC Universal just sold a Spanish language independent in LA for about $35 million. The amounts add up quickly -- but once you own the stations, you're no longer dependent on the mercies of your local cable monopoly choosing to let their viewers see your programs. You're probably talking about $200 million or more to get into the majority of the Top 20 TV markets.
Low Power TV (LPTV) is much cheaper than full powered stations, but LPTV stations can't compel cable and satellite to carry them. Nonetheless, for probably somewhere around $100 million, you could assemble a network of LPTV stations that would cover most of the top 100 TV markets -- probably 80% or so of the country's population. Your programming would be seen by anyone who has (or is willing to install) an antenna to receive the programming.
The most viable option would probably be a mix of full power and LPTV stations. A handful of full powered stations (carried by cable and satellite in their local markets) would anchor a progressive TV network, with LPTV stations in another 50 or 60 cities. The result would be something close to a national footprint, and might potentially reach a quarter of the country's population -- more, if you could convince cable/satellite subscribers in the LPTV cities to add an antenna. But you're easily talking about $100 to $150 million to put together the group of stations.
Putting together the programming would cost more. As would sustaining the initial operating losses (and remember that Fox News was a money loser for several years). Advertiser resistance to "radical" liberal programming would surely be another issue to overcome.
So, all in all, $250 million might be a reasonable figure for launching a progressive TV network.
Now, if one of the wealthy progressive billionaires out there would be interested in launching such a network, I'd certainly be excited as can be about the prospect. But it really would take someone with deep pockets and a lot of patience. And so far, the progressive movement hasn't really generated anyone like that. But if someone has the ear of Barry Diller, George Soros, Warren Buffet, et al...well, maybe you can talk them into it!