Scott Hartsman, CEO of developer Trion Worlds, explained why the shows cancellation does not affect the game:

Both the show and the game take place in the same not-too-distant future where humans and intelligent extraterrestrials are forced to coexist in the same town - the town called Defiance. 

The MMORPG was released in 2013, and despite mostly negative reviews, it has built up a loyal fan base that will continue to survive even while the show does not. 

The game was originally pay-to-play, but a year after release, it was changed to a free-to-play game to secure a lasting fan base. 

The show has been a bit of a cult hit since its beginning in April of 2013, just before the game was released. But, after a decline in audience, Syfy made the decision to end the show before the start of the 4th season. 

Defiance video game will continue in spite of show cancellation   Defiance - 80