True, in old forums we had ability to mark multiple posts as answer but here we can just mark one.
Why only one accepted answer?
Because only the person who asked the question can accept an answer, one answer will solve their problem. There might be several that are close enough, and for that, the community and the original poster can vote to indicate the rest of the users that the answer is also good.
Above details are documented here - https://learn.microsoft.com/en-us/answers/articles/25904/accepted-answers.html
If you want to change this, raise the feedback using Site feedback on top right
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Please don't forget to Accept Answer and Up-vote if the response helped -- Vaibhav