The cancellation was based off of underperformance in ratings, according to Fox.
Ratings mean something different to streaming services than broadcast networks; perhaps Lucifer could have a long life on Netflix.
The Netflix renewal means that fans will presumably get to see closure on the killer cliffhanger that ended the third season.
The cliffhanger was intended to stave off a cancellation, but that obviously didn’t work, as Lucifer was cancelled days before the finale even hit the airwaves.