1.) Commitment. Normarlly a network buys a (single) pilot episode, focus groups the hell out of it and goes back to the production company with results, which lead to changes in format, characters, etc. (Classic example being the orignal Star Trek pilot with Captain Pike wich was considered to "intellectual" and "liberal" (with a woman 1st officer) and transformed into the series we know). They buy one episode without commitment and work from there; they do not buy 100.
2.) A network buys a series to make a profit (like any other business venture); a network makes a profit by selling advertising. Advertisers buy time to sell a product to a market which has money to spend on said product. If the only people still interested in the franchise (particularly the PT era) agter the reception of the PT are people like the so-called "TF.N gushers", the only products this group spends its money on are Star Wars merchandise, which they will buy anyway. After spending their bucks on this stuff, they have no money left over. So, who would be the target for advertisers? Not beer makers, car makers, pharmaceuticals or any of the regulars.
3.) The reception of the PT. While Sith got a better reception than the first to prequels, this was largely due to the fact that it was the last one. You can only sell something as the last one so many times. Lots of people are just sick of the whole thing. And unlike a film in theaters where a film makes that same amount of money if ten people see it or if one person sees it ten times, advertising based broadcasting is entirely based on ratings--the raw number of people who see it at once. It is clear with cult films (which the franchise has become) that the huge numbers the PT generated at the box office were largely the result of the multiple viewings of a harcore fanbase, which does not translate well to the broadcast arena.