its all about money , according to the networks (TV stations) prime time viewing figures dip seasonally , an example,the Christmas period television viewing figures usually dip, so the networks get less money from advertizers (fewer eyeballs on the ads) so they carry over popular programs and run repeats during December . This month is known in TV speak as "March madness " a big national basketball competition which lasts a couple of weeks ... viewing figures for this are very high on CBS which has the contract to show the games so the other stations hold over their 'good stuff' since they would have lower viewing figures and CBS pre-emts its regular programing (NCIS etc.) to show these games
The regular TV season usually runs from October thru April , TV viewing during the summer is a lot lower (folks have other things to do outside in the good weather) and thats when we get re-runs and try-outs of new programs which may or may not make the Fall(Autumn) schdule .
Hope this answers your question .
just a note , yep lots of folks have DVRs and stuff but the advertizers dont count them as eyeballs for the ads since most people fast-forward thru them .
re. the sports channels .. they usually carry games nationally . If I want to watch a local game (Oakland or San Francisco, football or baseball) I would watch it on a local station , same as the folks in New York would watch their teams on a New York Station
as said before its all about money