The "terrorist org" never used those letters in the first place. The acronym in Arabic is "Daesh". ISIS is an English-language acronym formed from a (bad[1]) translation of the group's name. Think about it: why would a decidedly muslim organization name itself in a foreign language after a forgotten infidel goddess?

[1] Most western journalists rapidly moved off onto ISIL, which is a better translation (the name never referred to Syria the nation, but the geographical area better captured by "Levant"). Really only the right-wing sources continue to use "ISIS", presumably because it sounds more like a Bond villain's outfit.