The Bulgars actually split in the 7th century into a number of different tribes who ended up in different parts of Eastern Europe - the two most prominent groups were the Volga Bulgars (who settled in modern Tatarstan) and the Danube Bulgars (who founded what is now Bulgaria). The Volga Bulgars were conquered by the Mongols and largely assimilated into Turko-Tatar culture (but a related language, Chuvash, is still spoken in Russia).

The Danube Bulgars settled in what's now southern Romania and northern Bulgaria and quickly found themselves a minority ruling class in charge of a large population of mostly Slavic-speakers (but also lots of Greeks and Vlakhs). Eventually they began to speak the language of the region. It's really not an uncommon phenomenon. Just to name a few conquering groups who assimilated linguistically: the Scandinavian Normans in France, the Franco-Normans in England, Italy, and Ireland, the Manchu and Jurchen in China, and the various Turkic dynasties that ruled over Persia in the middle ages.

Interestingly enough, the founding dynasty of the Second Bulgarian Empire in the 1100s was of Wallachian (Romanian) origin and not Bulgar (though I think they spoke Slavic Bulgarian as their native tongue).