Bascially it costs money to repeat programs, there are actors to be paid and that's expensive. The TV channels will normally purchase a series to be shown, due to equity demanding a fair wack for their members services it's much more expensive to show UK television than it is to show American for example.
Therefore these companies know that people like for example "only fools and horses" (personally I think it's crap), so are happy to pay X number of thousands but not for something like Dear John as it would get a small audience (no matter how much you personally like it).
Here is an example of the difference in fees....
http://www.equity.org.uk/AboutUs/Magazine/defa ult.aspx?edition=13&page=12
Much cheaper to have A-Team on 24/7 than any quality UK TV.