It takes a lot of man power and money to get elected President. Just to get on the ballot in every state requires collecting thousands of signatures in each state in timely fashion and paying fees. It takes careful organization and lots of people working together to wage a successful campaign.
A third party has to compete with the two established parties and any other third parties. There is a large group of Republicans and Democrats who would never vote for anyone not supported by their party.
Even if a third party candidate won he as President , hewould have a hard time dealing with Congress unless the party was also able to elect a substantial number of Congressmen and Senators along with the president. To launch all the congressional campaigns requires even more money and organization. The major parties never stop working to sign up members and collect funds for the next campaigns. They maintain a full-time paid staff even between elections. There are no term limits on Congress and many Congressmen and some senators are sure of re-election year after year.
