Why do words ending with 'iq' have to have 'ue' after the 'iq'?

Answer 1

This is true of words borrowed from French, and words that are created using French roots. It is a convention in virtually all English words that the letter Q is followed by the letter U. However, as English borrows more and more words from languages like Arabic, there are likely to be more words in use that end simply in Q, like the name of the country Iraq. It becomes simply a matter of convention, and the adaptation of new words over time. English is particularly good at incorporating words from a great variety of languages.

Answer 2

Words ending with 'iq' followed by 'ue' are typically borrowed from French. In French, 'q' is almost always followed by 'u' in order to maintain the proper pronunciation of the 'q'. This is why words like 'piqûre' and 'équivoque' have the 'ue' after the 'iq'.