Here's the thing, it isn't always about how the show rates in the US. Ratings are just a part of the picture.
Other things that are taken into consideration:
- Production Costs. How much does it cost to producer per episode. for example, Firefly is a lot more expensive to produce than Two and Half Men.
- Ad sales. There has been a sharp drop in ad revenue on free to air networks. This hits their production budgets (see above).
- International Sales. Is the program selling to broadcasters outside the US? This wasn't always an important factor but as ad revenues have been dropping the importance of the international market, and the license fees earned has made it important.
- Domestic Syndication. Given all of the above, is there a strong indication that the series will achieve the magic number of 100+ episodes that will open it to the domestic syndication market? Is the content of the series such that it lends itself to syndication? There is a lot of money to be made in license fees and ad revenue through syndication.