I saw that atheism was closed because of lack of traffic. Is there need to worry about the same thing happening to this site? How is that decided?
It wasn't just a lack of traffic. From When Will My Site Graduate:
As long as the questions and answers are of high quality, and people get answers to their questions, you shouldn’t worry about the site actually being closed.
Atheism was closed because the traffic was abysmal and the answers (and questions) were simply uninteresting. There were many problem with the site, from an excessively narrow focus to pointless semantics. Skeptics has none of these problems.
In fact, so far, Skeptics has a lot of traffic for a site this young. It could be better - please evangelize the site to other skeptics! - but it is by no means near being closed.
Just to build on (and add an exclamation point to) Borror0's answer — AI was NOT closed because of a lack of traffic. The the AI blog post discusses that it wasn't the size of the site that was of concern; It was the conspicuous lack of topical discussions and experts.
AI’s root problems began almost immediately after launch and the traffic dropped precipitously from there. But the traffic was just the effect, not the cause.
Skeptics does not fall into that same camp.