As aaaaa said, Eastern Bulgaria was more active in the late stages of the National Revival (although the Revival itself began in the Western lands, Macedonia in particular) and was considered somewhat more “elite”. The Western dialects were also considered linguistically somewhat impure, due to the stronger Serbian influence on them. Also, most of the great Bulgarian authors from the 19th century had been Eastern speakers – Vazov, Botev, Slaveykov, Beron etc. Thus, it’s not surprising that there were four main language “schools”, all based on the Eastern dialects, of which the Tarnovo school eventually became the most dominant. Still, this was a very big question for our intellectuals from both ends of Bulgaria, both before and after the Liberation. And there were indeed Western Bulgarian writers (mostly from Macedonia, which still carried some weight as a cultural centre) who warned that the mono-dialectic basis for the literary language proposed by the Bulgarsko Knizhovno Druzhestvo (Bulgarian Literary Society, predecessor in Ottoman times of what would later become the Bulgarian Academy of Sciences) would eventually lead to the estrangement of the Western Bulgarians and that, thus, both the Eastern and the Western dialects should be represented equally. Unfortunately, the Easterners were more arrogant and succeeded in establishing a literary norm based solely on the Eastern dialects, even though (ironically) the capital of the newly liberated country was then placed in the Western lands, where it was supposed to act as the centre and unifier of the Bulgarian lands (Macedonia was expected to eventually be joined soon enough, and Sofia’s old name was Sredets anyway). This has led to an interesting mix, IMO, with the official literary language, based on the soft Eastern dialects, getting a bit roughed up by the capital area’s hard Western (Shoppish) dialect, to the point that now it’s us laughing at those soft-speakers in the East (well, they laugh at us as well, but that’s because we’re just so cool and they’re envious 😉 ).

Edit: If you can read Bulgarian, the first part here is rather spot on, as well as this one.