I am interpreting the question to mean:
Why are speculation and guesses consider off-topic as answers to questions on Skeptics.SE, when they are appreciated on other StackExchange sites?
There are two key parts to answer this question.
Is it an answer to the question?
On a site such as StackOverflow.com, an answer might say something like:
It sounds like you have the optimisation settings misconfigured. Check that the DLL has debug information included.
This is speculation; the answerer does not know whether it is correct or not. However, the person asking the question can use this information to check whether it really is the problem - they can quickly check the DLL and give a definitive response: Yes, that was the problem or no, that wasn't the problem.
So, even a speculative answer can be judged as correct or not.
On a site such as Workplace.stackexchange.com, where they are more likely to attract "Good Subjective" answers, there may be no definitive answers. The votes are a popularity contest, not evidence that the answer is "correct".
On the Skeptics site, the questions are not subjective (like Workplace.SE) and they are not easily tested (like StackOverflow.SE), so we demand that the answerer provide more evidence that the answer is correct. As Skeptics, we are acutely aware of the common mistakes people make with anecdotes, non-trivial logic and speculation about reality, we demand higher standards, like peer-reviewd journals.
It Defines The Community
A key aspect of defining a community is to define what you have in common and, just as importantly, what it off-topic.
For most StackExchange sites, the domain is very specific - e.g. students of the Japanese language. On the other hand, Skeptics.SE is troublesome, because the topics of discussion are very wide - everything covered by the Dewey Decimal System is on-topic. Instead, we are united by our desire to cut through the nonsense found in most communities, and get as close to the truth as academic study can get.
We could allow speculation in answers, just as you could play poker at a book-club, but to do so would discard the purpose of the site and ignore the heart of the community. It would quickly become just like most other forums on the web, and equally as pointless.