For the past several decades members of Congress have had to “pay to play”. If they want to be on a committee and do more than just vote at roll call they are expected to raise funds for their political party. The easiest way to raise money is to establish alliances with lobbyists who represent wealthy individuals or corporations. This promotes corruption.
The two major parties pass state laws making it difficult for a third party to gain and maintain ballot access. To establish a political party in California requires 60,000 residents to register for the new party or more than 750,000 valid petition signatures. The bargain rate for signature collectors is $2.50 per signature. Typically because of moves, unreadable names and addresses 25% of the signatures are struck by state election offices. It would cost $2.5 million to establish a new party in California.
It is easier, faster and less expensive to influence a major party from within than it is to establish a new national political party.