When you can only vote for one or the other it causes the voting system to devolve into a hyper-polarized us-against-them dynamic. In order to move beyond a two party system we need to be able to show support for multiple candidates.
Voting your conscience can backfire. This is why so many people are pressured to "vote lesser-evil"
Our current system is wildly inaccurate when there are more than two candidates in an election. Voter blocks who support more than one candidate can split the vote and cause another candidate to win unfairly. This is called the spoiler effect and it can cause a candidate who is opposed by the majority to win.
If you aren’t in a contested area or if you are in the minority your vote doesn’t make a difference. This is a huge reason why so many people don’t vote.