My guess on this is two-fold: Sci Fi is trying to cut expenses by dropping SG-1 because Tapping, Judge and Shanks have got to be pulling down pretty large salaries per episode codsidering that 2 of the 3 have 10 years invested in the series and one has about 9 and sort of a half seasons invested. Depending on their contracts and SAG rules, they're likely hauling down very nice salaries, but would probably like to move on in their careers. Browder, Black and Bridges are new to the show, but Black and Bridges are known names and probably are getting a bit more than scale, I would imagine.
The second issue: stories. After 10 years and saving Earth oh, I dunno, 10-15 times, from the bad guys out there...what do you do after you've defeated alien parasites bent on galactic conquest, replicating "bugs" bent on galactic conquent and now ascended beings bent on galactic conquest? You start running out of decent ideas after 10 years and 215 episodes and have raised the stakes with every "bad guy" you've invented.
I'm sure that some of the cast may go to Atlantis (Black would make for a fun addition to Atlantis, as would Browder) and the others may show up from time to time as well.
Between Atlantis and BSG, I'll be okay with it, as long as those hang around for a few more seasons. And SG1 will undoubtedly last in syndication for a very long time to come (maybe on UHD even!).