Their ratings have fluctuated. Things started off strong. Then they lost over half their viewers with the vulva garden episode. That big guest star packed episode gave them a nice bump, but then the orgy episode caused ratings to drop again. Raunchy material seems to kill off a portion their audience.
https://tvseriesfinale.com/tv-show/what-we-do-in-the-shadows-season-one-ratings/
This chart only shows live viewers. The series does better with DVR viewers than live ones. The network undoubtedly took the DVR audience into account when they decided to renew it.
Tilda's partner used to live with Taika Waititi. WWDITS and Tilda's movie Only Lovers Left Alive came out at the same time, and she told them they should team up their vampires for something in the future. That's what brought about the initial idea for it. Then when the show came about, it was the perfect opportunity to bring their vampires together. Since they had a big name like Tilda willing to do their show, it was a lot easier to get other people to agree to it.
