It is really difficult to pinpoint the exact reason for a community like Renderosity to get inactive.  It must be because of several reasons and not because of poor photos or bad comments or what who is doing right or wrong or not doing etc.

I for one was quite inactive in 2011/2012.  Some life changing events, challenges and work held me up.  I am taking photos as a hobby and am having a full time job as an IT Manager for a government institution.  This in itself causes me to have very limited time to spend on social networking sites although I am still trying to participate because of what it means to me.

I post here because I WANT to and I like viewing the photos of people who share their thoughts and photos here.  Many might not be of a professional standard but that is not what Renderosity is suppose to be in my honest opinion.  For me it is a community where you can share your photos, get more ideas and inputs from other people.  It is about making friends and sharing things you like with them. 

I have a photo page on FB as well and to be honest I don't have people leaving comments there like they do here.  You might get your occasional 'like' and that is about it.  At least here at Renderosity people try to comment, even if it is to pass on critique about the shot you took.  You also learn things from other people sharing their work here since the technical side is also discussed many times. 

In short, Renderosity is what YOU as a user make of it.  Nothing more and nothing less.