Since the first series, Stargate _SG-1_, used the original film as its launching point, Dean Devlin tells Variety that the only way to go back and make the film trilogy that they want is to reboot the entire thing.
Even if Dean Devlin and Roland Emmerich wanted to ignore the TV series and make a direct sequel to the original Stargate, all the characters would be 20 years older and, for whatever reason, Devlin says that wouldn’t work with the story they want to tell.
When the original Stargate came out in 1994 it was always meant to be the first part of a film trilogy.
However, after MGM obtained the rights to the property, they went on to create a number of TV series without input from the film’s creators, Roland Emmerich and Dean Devlin.