DavRob60's answer is basically right, but I wanted to take this opportunity to write up a more detailed answer that applies to more than just SciFi.
What we've done before
There are several different types of contests that tend to pop up on our sites. They often overlap, but in the most broad terms they are:
- What I like to call "internal contests" - things like tag clean-ups or topic of the week contests that are primarily aimed at engaging the existing community on the site. The goal here is to do something fun to break up the "monotony" of the day-to-day. These are typically initiated by the site's community rather than a Stack Exchange employee.
- External promotions. For example, Answerama - an event that was publicized externally and aimed at drawing in new users who didn't already know about the site.
- Content generation events. These are typically internal, but tied to some special event. The most common example are promotions like AnswerSwarm on Arqade.
- Some combination of the above. Skyrim vs Modern Warfare on Arqade sort of fit into this category. It got a lot of attention from existing site users, but it was also publicized externally.
The rest of this post is going to be mostly about types 2 and 3 (and 4).
How it all went down
We've run a bunch of promos and events over the last couple of years and one of the things we learned was that extrinsic motivation in the form of expensive prizes doesn't bring out the best in people. This is not really surprising; swag is great and all, but the best incentives for growing a site and participating in a community tend to come from intrinsic motivators.
Now, in order for intrinsic motivation to kick in, someone has to know that our sites exist in the first place. Externally-facing contests seem like a great way to spread the word, but unfortunately it doesn't tend to attract the good kind of attention. I'm sure you folks remember what happened during Answerama - the event was promoted on Facebook and that brought in a flood of extremely low-quality posts from people who didn't know how this site worked and also didn't really care to learn because they were here to put up a quick question and enter into a prize draw. Personally, I can't really blame them: if I want to enter into a contest, I sure do the absolute bare minimum that I have to in order to maximize my chances of getting something I want out of it.
Despite an influx of low quality content, these contests brought new eyeballs to the site. Some were more effective at that than others. We did have some issues with user retention where many people who showed up for the fun didn't stick around to post much after. Not all of them went like that, of course, but what we discovered is that even contests of a similar type can have different results in terms of encouraging site growth and publicity.
We love rewarding our existing users for excellent participation in these contests, but even there there's also a non-negligible logistical overhead. Once we start doing things like buying folks TVs, we very quickly get into a situation where we have to keep track of what we give out for tax purposes, participants have to fill out tax forms, and it goes downhill from there.
So some of these contests weren't terribly effective. Can't we still run them once in a while? Surely some will have great results!
Well, that's certainly true and we definitely know more about what can and is likely to work now than we did in the past, but the results are never guaranteed.
Unfortunately, part of the problem here is that most of the people who headed up this effort at one time or another are unfortunately no longer with Stack Exchange. The rest of the community growth team have either moved on to different roles or are busy trying to wrangle 100+ sites. Maybe in the future, as the team grows, there will be more room for SE-driven promotions and contests. We'll see. In the meantime, we are prioritizing projects that hopefully benefit everybody.
We're also still experimenting when the time allows and trying out other types of fun events and contests as time allows, but we're trying to avoid things that amount to paying folks for new content for our sites.
What does all this mean in the end?
We still love you guys. This and every other community we have in our network. If you have an idea for something cool to do on SciFi, bring it up on meta, hash out some details, and we'll be happy to back you up with some sweet swag.