Blow jobs are a big niche in porn since a lot of guys can't get them. Teen blow jobs are even better since not too many older guys can get oral sex from an 18 or 19 year old girl. Oral sex and cumshots and cum shot facials are also very pleasing to the web surfer eye. Pretty girls like to suck real big cocks too. It gets them off.
Teens are usually more pretty since they are younger.