The Muslims changed it after the fall of the Byzantines who had named it Constantinople after Constantine, the first Roman rmporer to convert to Christianity.
I did not look this up so am open to corrections ..just a hunch.
Actually the city’s name was only changed in the early 20th century in an attempt to create a new Turkish national identity. Before that the Turks kind of saw themselves as overlords of native peoples they ruled over. Their ancestors lived in tent cities and they would tax the native city and towns people. When the Ottoman Empire fell this created a need for a new type of Turkish identity which saw them now declaring they were natives of the lands they occupied.