HL attracts a type of crowd that as pubbers would be less likely to jump into 6s.
If you go to your average community pub you will see a lot of offclass "mains" that spend hours in those servers playing engineer or heavy or something like that because those are the classes they like. I'm sure a lot of you guys have been that player before. If HL didn't exist those guys would be less likely to try competitive at all, because it implies learning at least one class that they're not comfortable with or don't enjoy. I'm not saying that's a good mentality, but it would happen.
At least with HL they can get used to the basics of playing with a team and communicating while playing the class they like, so it all makes a little more sense in their heads. And there's also the aspect of 6s being seen as really "tryhard" and unforgiving from the outside, whereas HL is more casual and more friendly in that way.
TL;DR if it weren't for HL the scene as a whole would be much smaller and that includes 6s