Both "На Донбассе" and "В Донбассе" are correct ways to say "in the Donbass" in Russian. The choice between "на" and "в" depends on the context and can vary based on regional dialects.

The use of prepositions in Russian is not always straightforward and can be influenced by historical reasons, regional differences, or just individual preference. In this case, both "на" and "в" can be used to indicate location within a region, such as "на Донбассе" or "в Донбассе".

As for the question about why "На Кузбассе" is not commonly used, it's likely due to historical and regional differences. Each region may have its own specific preposition that is most commonly used to refer to being in or on that particular place. In the case of the Donbass region, "на" and "в" are both commonly used, but in the case of Kuzbass region, "в" would likely be the more appropriate preposition.