Because each state runs all elections including Federal ones and the administration is mostly at the local level there are enormous problems due to the variation of laws and practices throughout the country. For a 3rd party candidate to get on the ballot there are 50 sets of rules and deadlines and they are affected what happened in a previous election and the status the party.
Then there are are the problems and variances with voter registration, how one votes absentee, overseas ballots - different in each state. Access to the polls on election day, what is printed on the ballot, whether ID cards are used (to protect against ID fraud for which there is essentially no evidence for).
And then there are the voting machines and how the counting is done with lots of proprietary software that no-one other than the vendor has seen and varying policies about how improperly scanned ballots are handled and how recounts are done. Remember 2000.
The electoral college violates the one-man one-vote system since each state gets 2 for their senators + 1 for each rep so the ratio of number voters to electoral votes is skewed.
She is a big fan of IRV (instant runoff voting) as am I which would make 3rd party candidacies make much more sense and would be especially good for primaries. (The state senator primary election in my district is on April 13 with 6-8 candidates running. The winner could have 20% of the votes - my congressional rep (Capuano) for life won his primary with 23% of the votes - and voter turnout in primaries is abysmally low.
She has lots of suggestions about various cures to these problems but because the two existing major parties essentially control everything makes it unlikely that anything will ever change. Considering how much dissatisfaction there is in the current political system, maybe there will be some improvement in the future but I'm not hopeful.