Yep. If someone is legally selling booze to adults, they incur a serious risk to their legitimate business by selling to minors. The risk/reward just isn't there.
But with pot, the sale is already illegal. What difference does it make if you're selling to a minor?
MJ is probably also easier for a teen use and not get caught. A small bag would probably be easier to hide than a six pack. Eye drops and a breath mints can help mask MJ use, but drinking can be more problematic. Young drinkers tend to get sick, pass out, and sometimes end up in the hospital or worse.