I think not having to pre enter changes the demographic to start with...
I don't think that's the reason; young people are pretty comfortable with pre entry when it's electronic... I don't think there are many races left where the only way to pre enter is by post (although admittedly there are some, and they are all fell races...)
There's a good week-night scene up here in the Peak. Sometimes you'll get lucky and the field will be weaker, but more often than not (especially before Shef Uni breaks up) the top 10 will be pretty packed, good hard racing with nowhere to hide.
I generally refuse to enter a short weeknight race if it's pre-entry only (you don't want to get tied in to a traily race), but some races have had to go this way after getting some ridiculous numbers of late (sunny evening, jogging clubs heading out en masse from both Sheffield and Manchester ways)