In Spanish it's actually spelled coronel

According to the QPB Encyclopedia of Word and Phrase Origins:

"The confusing pronunciation of colonel is due to several errors. Colonel comes from the Italian colonello, which meant 'the officer who led "a little column" of soldiers at the head of a regiment.' This became the French colonelle, meaning the same. So far, so good. But the French word became corrupted to coronel in Spanish through faulty pronunciation and was introduced to England in this form. Until the 17th century the word was spelled and pronounced coronel, and then its spelling was changed to the current colonel. Yet despite the efforts of teachers and pronouncing dictionaries, people refused to pronounce the word colonel. Early habits were too great to overcome and they continued to pronounce it like coronel, the experts finally giving up and accepting this as the standard pronunciation."

So it is the fault of the Spanish. You woulda thought the Japanese had a hand in it, but no.