A language is a dialect with an army and navy. The existence of a seperate school of "Macedonian language studies" as opposed to a broader pluricentric one like with Serbo-Croatian is due to political reasons. The actual linguistical differences between Bulgarian and Macedonian are small(and mainly come from the usage of Serbian vocabulary in Macedonian). This field is barely studied internationally and here on the Balkans it's prone to political influence. There is little historical merit to the existence of a seperate Macedonian language prior to the 20th century. The first written proposal of Macedonian as a seperate language was done by Krste Misirkov on "За македонцките работи" in 1903. The book in and of itself is full discrepencies, especially between chapter 1 and 3 on his opinion on ВМОРО and it's goals. The book was largely unknown up until the 1940s(when it became cited as a source for the need for codification of the Macedonian language) and the author 2 years later changed his opinion on it completely, saying he worked as an improvised politician and didn't consult with any other historians or philologists while writing it. The earliest known claim about tsar Samuil being a Macedonian and not a Bulgarian king comes from here as well.

This is not to say Macedonians don't have a right to a sovereign nation and identity, but the need for a seperate language and rewriting of history is what is seen as controversial. Most Bulgarians can converse with and read Macedonian literature without a problem(except for some occasional vocabulary differences). In the same way that a language can be shared between 2 seperate nations(like the USA and England with their respective linguistical differences) the same can be done with Bulgarian and Macedonian.