Maybe because people lose interest in VCE as they have seen more of life/studied to a higher level then.
This. Nova, this site will always have a high 'drop off' rate because of its target audience being VCE. I don't honestly think security, spamming etc. is a major reason people leave, it is that they see no benefit in staying as they have completed VCE.
You can suggest expanding university forums but realistically it's not going to do much, because of the small uni community on this site. If you take all the uni students here, very few would be doing the same degree with the same subjects at the same uni, so setting up a VCE-style question board wouldn't really work.
In my opinion, only a select few will stay as they progress through uni/tafe etc. and this will always be the case. Theres not a whole lot the site can do. It's aimed at VCE students, and they are going to make up the majority of users. My 2 cents.