I don’t recall the originals playing on tv after 1997. And they probably had not been on tv in the U.S. for a few years prior, building up to the 20th anniversary and so as to not cannibalize sales of the One last time OOT video release. Spike Tv had the exclusive U.S. basic cable rights to the OT and the prequels from 2008-2014 and that was always the special editions.
The SE’s probably made the rounds on HBO and other pay cable networks before the Spike deal kicked in.

All the films (up to Rogue One) have been on TNT and TBS quite a lot lately. I presume they paid a hefty fee for exclusive basic cable rights.