New TV shows may start out with an intended number of seasons, but when a show proves its popularity and is greenlit for more, most shows continue past their original plan. Getting a new show greenlit is a challenging task for creators who may shop the show to several networks before getting a pilot approved. Even with a pilot episode of the series recorded, this is not always a guarantee that the show will be picked up for a whole season. When studios pick up a show and approve a season or occasionally, multiple seasons, writers need to plan carefully to ensure the season ends in an engaging way that will draw viewers back in and set up further seasons.

https://screenrant.com/shows-lasted-longer-originally-planned/