Why the Bulgarians rename the Old Slavic language to “Old Bulgarian"?

Why the Bulgarians rename the Old Slavic language to “Old Bulgarian”? The chronology is as follpws: Moravian king Rostislav decided to use the Slavic language in the churches of Moravia so he applied to the Byzantine emperor to send him some knowledgeable people to do the job. Kril and Metodius from Solun were chosen since they were highly educated and knew the Slav language because the Slavs were principal inhabitants of Solun and its vicinity. They translated the required clerical books into Slav language and went to Moravia. The Germans did not give up easily so Kiril and Methodius had to face the Roman Pope and to justify the use of the Slavic language which they did and the Pope approved. The new Moravian king returned the Germans so Kliment and Naum went back to Macedonia which was ruled by the Bulgarians. The Bulgarian tsar wanted to get rid of the Greek language in the churches so he welcomed Kliment and Naum and gave them an opportunity to continue with their job but this time for replacing the Greek language with Slav language. As it happened over history the Bulgarians started to name the Old Slavic to “Old Bulgarian” so when you are using dictionary of “Old Bulgarian” you are suppressed how similar the words are with the present Macedonian. At present the Russian is the most similar to the Old Slavic, so are the Bulgarians brave enough to tell the Russians that they are speaking Bulgarian? Can Bulgarians tell the Serbs that they are speaking Bulgarian only because they can find similar or the same words in the “Old Bulgarians” dictionary? Can we easily overlook the crucial fact that that language the Old Slavic was taken from the Macedonian Slavs? Is it not right that language to be called Old Macedonian?? That language was the spoken language of the Macedonian Slavs. Now who is the thief???