Sometimes it just suits the story itself to have more than one season.
Some reasons may include; the seasons taking place at different time frames for example S1 in high school and S2 in college or the main characters changing for example, S1’s main’s kid take over for S2.
Seasons can also provide breaks for the author and let readers anticipate and be excited for what’s to come next in a way that’s a bit different to a cliffhanger because they’re anticipating a whole new section of the story, not just how a cliffhanger plays out.
It’s hard to explain tbh, for me it just fits and feels right to have more than one season if the story is a bit longer, especially if there are different things to expect from each season. 
The way I see it, it’s like a report with paragraphs, except each season is a new paragraph and contains slightly different or completely different content from the last and it’s also neater as opposed to one large paragraph. 