Why don't more shows jump networks?

So it looks like Out of Practice won't get picked up next season, but why doesn't CBS/Viacom dump them into its new CW network? Out of Practice averaged a 11.6 (#32) which is pretty respectable for a new show. While it may only be considered a minor success, it would be phenomenal numbers if it were on UPN or WB. I realize that even if it transitioned, it would probably lose a lot of audience due to CBS being the #1 network, but if it even averaged a 4.0 on the CW, it would be a big success.

You can find the primetime ratings here.
http://www.hollywoodreporter.com/thr/television/feature_display.jsp?vnu_content_id=1002576393

Just a few random ratings notes. For as horrible as it was, Courting Alex averaged a 11.2 The Adventures of New Christine beat Out of Practice with a 12.5

The highest rated shows for the WB was 7th Heaven (#111 5.2) and the highest for UPN was America's Next Top Model (#113 5.0)

Here are a list of highly acclaimed shows that OOP beat out:

Law & Order (#35 11.2)
My Name is Earl (#40 10.9)
Boston Legal (#46 10.3)
How I met your Mother (#51 9.5)
Prison Break (#55 9.2)
Will & Grace (#61 8.7)
The Office (#67 8.0)

Once again, I'm not saying it should be headlined as CBS's #1 show, but it seems to be a solid performer, and a very safe bet especially considering it has little star power.