Originally Posted by knope2001

Back when Midwest was formed and assigned their code, there were few two-letter combinations available to choose from. YX at least had an X in it, as the airline was called Midwest Express.

Numbers were being introduced as part of the code back then, but only in the first character such as 7R or 3A. They might have been able to get a code like 8M, but many new airlines in the US avoided those like the plague because they were perceived the domain of come-and-go upstart commuter airlines. So lots of new airlines got codes with letters not associated with the carrier or its name.

In subsequent years, codes were issued with a letter in the first position and a number in the second (like F9 for Fronitier) and the stigma faded to nothing. But airlines like Midwst (YX) and America West (HP) held on to their original code.