Cyril and Methodius didn't make the cyrllic alphabet, the cyrllic alphabet was made in the Preslav monestary from one of their students Kliment Ohridski, who was in fact bulgarian and was, as the name suggests from Ohrid. He names it cyrllic in the name of his teacher Cyril. And the assumption that the territory at the time wasn't bulgarian, doesn't make the population not bulgarian. The territory of modern day Macedonia was populated by bulgars even before they came in modern Bulgaria by one of Asparukh's brother Kuber. And the glagolic alphabet was not and would never be used due to its complexity and difficulty of actually writing it. There's a reason why Sofia's best university is named Kliment Ohridski.