Why the Triassic period has a low biodiversity- Mesozoic Era: The Age of the Dinosaurs?
Presumably the massive extinction at the end of the Permian took some time to recover from. Admittedly, extinction events provide lots of opportunity for diversification (lots of unfilled niches that the remaining life can move into), but it does take time to happen. Basically, the Triassic is the development period for the later diversity in land life that was the Jurassic.
The Permian ended with a massive extinction event, possibly brought on by volcanic eruptions and global warming (but we don't necessarily know the reasons). That left fewer species living, and life was hard for them (a) because so many other species had disappeared and (b) the climate was arid. A further set of extinctions occurred about 3 million years later, again perhaps due to animals being in habitats that became too hot.
The Triassic follows the Permian which is known for the mass extinction of both plant and animal species. While a few species survived the Permian it would have taken thousands of years for life to recover.
Not sure if it's because of the Mass Extinction (from Permian and Triassic period) which caused low biodiversity. Correct me if wrong-