Trump sought and actively tried to subvert constitutional government and overturn the results of the presidential election. And what he could not do through the arcane rules and procedures of the Electoral College, he tried to do through the threat of brute force, carried out by an actual mob.
If you can’t cough up the fee (typically $100 or less for most state and local offices) and get several dozen people to sign your nominating papers (varies somewhat by state and office) you’re wasting your time running in the first place.
The big reason we don’t see third parties doing well in the US is that the bulk of the country uses first-past-the-post general elections. These mean that a vote for a third party candidate hurts the people most apt to implement the policies you care about.
Three cases:
Case 1:
D - 10 votes, R - 9 votes, I - 0 votes
Democrat wins
Case 2:
D - 9 votes, R - 9 votes, I - 1 vote
Tied election and a coin toss used to decide winner
Case 3:
D - 8 votes, R - 9 votes, I - 2 votes
Republican wins
Case 4:
D - 9 votes, R - 8 votes, I - 2 votes
Democrat wins.
There are systems that solve the spoiler effect, such as:
In each system, the candidate preferred by the most people (usually a majority) wins.