There is no actual rule or anything as to why "Q" is followed by the letter "U" in the English language, so I don't think you'd find an absolute answer to your question. It just is, lol.
Buuuuut I've studied the English language quite a lot on my free time and I've seen (in some places that I can't remember) that in Latin, the letters 'Q', 'C', and 'K' had their respective phonetics but the letters 'U' and 'W' both had one sound... Which was 'V'. For an extensive period of time, our "QU" had been known as "KW", then "QV" for the most part. Afterwards, 'U' and 'V' were distinguished and we began to use "QU".
English doesn't have its own words with just a "Q" and no "U" following it, though. We just have borrowed words, like "cinq" (derived from French), "qi" (Chinese), or the "qwerty" keyboard (which I wouldn't really say is a word, but it works!)That's about all I know :P