[asveikau]

X in Mexico or Quixote used to be pronounced as "sh". I think X is still used for that sound in Portuguese or Basque or Catalan if I'm not mistaken.

In the case of Mexico they were transcribing a word from indigenous people.

In a bunch of Spanish words coming from Latin, it was often from Latin double S. Eg. páxaro, from Vulgar Latin passarum. I can also think of cases of this sound coming from Arabic, eg. modern ojalá was once oxala which is a cognate with "inshallah", note the sh sound.

Sometime around the 1500s this sound changed to /x/ and they changed spellings of a few different sounds whose values had shifted over the centuries. X had merged with a sound they spelled with J, derived from Latin words with J in them.