I've made 682 votes so far this year, roughly about 4/day. However, I've trailed off considerably over the last 2 months, both in voting and in activity.
For reference, I asked a similar style question here last year:
Sure, we like the IDEA of voting, but we are poor in our actions
as a follow up to: Is ServerFault doomed? Not if we all vote more!
So to answer your actual questions for me:
Why don't you an active member of Server Fault bother to vote on
Overall, I try(tried) to. But I also tend to only vote on answers that I actually know something about and can agree that the answer should (or shouldn't) work for the OP. Which is different than my voting style for questions, which is based more on the content, wording, and overall value of the question here on SF.
So, what's changed? Well, honestly it's a combination of not being as active on the site itself over the last 2 months, as well as a sort of lack of motivation/incentive to vote period. It's not a "what's in it for me" attitude, but the focus for me on SF has been more through active participation in the Comms Room then it has the main site.
What can people do to make you want to upvote their correct answers ?
Well, I don't think the people that are answering the questions can really do much of anything. My motivation to vote or not vote isn't necessarily based on the answerer but on the answer itself. Yes, I do tend to upvote answers from well-known members, especially if the Question/Answer comes up in the chat room, but the deciding factor is more along time/self-effort to even look at the Q&A main site. When I have time for SF, I either do it in the Comms Room or on the main site. Often chat wins out. If I'm on the Main site, I will first look for questions I can answer, then look at questions, then look at answers. Often always in that order. Selfish? Possibly...
Here's where the site involvement frustrations lie, IMO. You have a site that uses voting/points/badges as incentives/fun. Good enough...and most people will gravitate to something that even appears to be handing out points/awards/etc. Take away the voting/badges/chat/privileges/etc. and the site becomes very dry and almost certainly would become a giant question dump with very few active "answerers" sticking around just for kicks.
So, what's the answer to the riddle? No idea...we've discussed it numerous times and I don't know if there is a magic formula. Site marketing, fresh users, points contests, the funny little things like the unicorn points, etc. all get tried here and elsewhere, but often they are only temporary boosters if that.