Probably because its dangerous to democracy to silence people who question the integrity of elections. It sends the impression that there is something to hide. And people who suspect that the election was stolen will take this as confirmation of that suspicion. Since they won't be allowed to use that platform to raise their suspicions, no one will be able to respond to them to allay their suspicions. Then as time goes on confirmation bias will lead them to become entrenched in this belief. Eventually this belief (that the elections have been stolen) will lead to the perception of a loss of legitimacy for the government, and consequentially public servants will find it more difficult and dangerous to do their jobs.