Well I don't exactly have a list of every event ever hosted printed out and in my hand, but I do think it's possible that the types of events aren't helping. If every event is a minigame, boss monster hunt, or wildy fight, people are going to get bored with it quickly. It's basically a constant rehash of stuff people have already done a hundred times over both in groups and by themselves. That can tend to give people the feeling that the events aren't creative or significant enough to warrant their attention.

How about trying some events that don't rely on minigames, boss monsters, or the wilderness. Try some thing that could be done in F2P. I know simply having an event in F2P can give a boost to attendance for that event no matter what kind it is. Perhaps things like fire fests, freak trains, attack of the nulls/clones (<- that one I'm a little iffy on), find me's, scavenger hunts, area NPC annihilation, or maybe make up a story that would fit into RuneScape and have the event further/resolve the story.

There's all kinds of things you could do if you're willing to think outside the box. Hell, I once thought up an event inspired by the movies Resident Evil and Lethal Weapon 2. It was an F2P undead hunt with us all in guard uniforms which I dubbed "Demise of the Undead". I already hosted it elsewhere so it may not be such a great idea to reuse it, but that was just to give an idea of what could be done. If you already do hold events like that and I simply haven't noticed, why not more often?

Anyways, these are just suggestions, but I think it could help (if only a little).

Yes, we had one on f2p recently. We discussing about f2p also.