It's because Republican's and Democrats have huge funds. Most potential 3rd party candidates just go with either Republican or Democrat depending on what there opinions are closest too so that they're taken seriously.

TTG_jrj109z wrote

False. A third-party canidate has already been elected. Millard Fillmore (1850-1853) of the Whig Party was the last third party candidate to be elected president.

This statement is actually incorrect. The Republicans did not exist at the time, the Whigs and Democrats were the only two parties when Fillmore was elected. The year following the Republican party was formed by Anti-Slavery Whigs turned Republicans. I cannot recall who specifically