It's only newer MMA fans who got into it recently. Older fans for the most part have the utmost respect for the sweet science.
Joe Rogan said something awhile ago, "[MMA] is the sport of fighting, boxing is the sport of punching" which I agree with a lot. Some people would take fighting over punching. I personally love both sports, but I know plenty of people who can watch MMA and can't stand boxing. In many opinions MMA is more exciting and entertaining than boxing, and I believe is that is why most people bash it; compared to MMA it might not be as entertaining in certain opinions.
Good answer maybe they bash it because they just don't like it. That doesn't make them ignorant necessarily or we would all be ignorant for not liking something or another.