There are a few possible reasons for this:
* Lower demand: The show's popularity may have declined after the first two seasons, leading to lower demand for DVD releases.
* Rights issues: The rights to distribute the show on DVD may be held by multiple companies, making it difficult and expensive to release the later seasons.
* Production costs: Releasing a DVD set can be costly, and the studio may not have deemed it profitable for the later seasons.
If you're interested in watching Season 3, you can usually find it streaming online on various platforms. However, the availability may vary depending on your region.