I think the club is conservative about it because there will be a day when we don't celebrate Queen's Birthday anymore. For most people (actually, probably all), Queen's Birthday means nothing and it's just a day off. Yeah, it might be the only game on the day, but you can't expect it to be any bigger than any normal Friday night game.
There's no real history in the match outside of the Melbourne and Collingwood rivalry, which has essentially been dormant since the 60s. No one cares about what Queen's Birthday is.
I don't think if the club promoting it any more would help any apparent crowd drops. I mean, how coincidental that they drop when we're "winning" wooden spoons. The crowds get to as big as they're going to get, dictated by the performance of both teams.