Well, Unova has been stated to be far apart from other regions. It would make sense that not a lot of old species would carry over.
Unova was also a sort of reboot for the series, with around 150 all-new Pokemon, like Kanto. Of course, old Pokemon were brought back in the sequels, so it wasn't exactly a reboot for long. A lot of Unovan Pokemon are thought to be Kanto counterparts as well (Bouffalant to Tauros, Sawk and Throh to Hitmonchan and Hitmonlee, etc.).
Hope I helped!