You posted this under Legacy products, so not the best place to look for a response.
What channel are you watching this on? If it's something besides The Roku Channel, then you need to ask whomever offers the channel you're watching. If it is TRC, remember that when Roku gets permission to stream content, they are often limited to the number of titles they can stream. For TV series, often they do not have permission to stream all seasons, and sometimes not even every episode within a season. Whomever owns the content can restrict what is permitted. Will additional seasons become available later? No way of knowing. Roku doesn't announce changes in programming in advance, and it's possible the content owners don't want to release additional seasons.
Dan Ultra (4640), Ultra (4670), Ultra (4800), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.