Did we forget how to pronounce them, or did society accept mispronouncing them then when they were in larger use? English’s rules of pronunciation may be loose, but they still exist, and those “incorrect” pronunciations follow the rules.
This is one way that language grows and changes.
The French insist on pronouncing foreign words in their foreign pronunciation, but they can do this only in part, and they wind up pronouncing them with a French accent.
The English do just the opposite. They give foreign words an English pronunciation.
Only the other hand, Don Juan is given a French pronunciation in Bizet's Carmen.