Today we expect a peaceful transfer of power when Americans vote to change political parties in a presidential election. Why were people unable to make this assumption in the election of 1800?
The reason why we expect this is that we have had all these years of peaceful transitions of power. We have been having this happen for over 200 years and so we have come to expect it.
However, in 1800, this had never happened before. There was no tradition of letting power go from one party to another. People had no reason to assume that the transition would be peaceful.
What is more, this was the time of the Alien and Sedition Acts. The Federalists had passed these laws that essentially prohibited any dissent against the government. In such a situation, why would you expect that the transfer of power would be smooth and peaceful? It had never happened before and the party in power was suppressing the speech of the opposition party. It would have seemed rather unlikely that the transfer would go well.