-- One of the reasons WWE likely decided to go with FOX over a streaming service like Amazone or Facebook (one of which offered even more than $200 million) is because traditional television on a station like FOX is better for both creating new fans and retaining old fans.
-- Roughly half of the current Smackdown audience is over 53 years of age and it is believed it would be this demographic that would lose touch with the show if it was taken off of traditional TV. It is also good news for FOX because a Friday night timeslots are difficult and this older demographic is less likely to go out on a Friday night instead of staying home and watching TV.
-- Another major reason as noted before is FOX's promise that they would heavily promote WWE programming on its other sports broadcasts - NFL, College Football and MLB games. Specifically, FOX will advertise Smackdown on its Thursday Night Football game and their plan is to push NFL on Thursday, WWE on Friday and College Football on Saturday as three consecutive nights of sports/entertainment programming.
Source: Wrestling Observer Newsletter. Subscription information & prices available at f4wonline.com