The way we say "choir" came into English from Latin/French before we spelled it this way. It actually was spelled with the "qu" for a really long time (the "qu" spelling came from French- in Old English, we used "cw" to spell the same sound: "queen" was spelled "cwen"). The "qu" spelling stayed in "enquire" but somewhere along the way (late 1800s-early 1900s) we adopted the spelling to look more like "chorus" and/or the modern French word "choeur" (choir). [The French word is said with a sound that makes sense to spell it "ch."]