Because, like it or not, Joe six pack and Jane public equates Regional pilots flying RJ's as being less experienced then those flying larger aircraft. People assume, right or wrong, that the larger the aircraft the more experienced the crew. The think those pilots are making more money and hence, have more experience because of that which means they must be safer.
Now having said that, I've known heavy jet pilots I wouldn't let drive my car across the parking lot and I've flown with extremely competent Regional/commuter pilots. I know it sucks having people judge your experience level based on aircraft size or your age. It's human nature. We want our pilots, surgeons, priests, Company CEO's, etc...to have gray hair and plenty of experience.
When I was a 24 yr old commuter Captain on a Metroliner I got the stares and comments about if I was old enough, experienced enough, etc...to fly them to their destination. There is a cure for that. It's called Father Time. It catches up with all of us if you live long enough. And with time comes life experiences and hopefully good judgement which probably came from little experience and bad judgement.