Because people who do not have the approval of party power brokers and wealthy donors are effectively locked out, leaving only people of which the power brokers and donors approve.

The primary requirements of the power brokers and donors are that the would-be candidate has no principles that might interfere with them getting whatever it is that they want but have enough skill and/or charisma to project the illusion of principles to the voters. They want someone they can count on to exclusively serve their interests while maintaining enough of an appearance that they serve the interests of the people to win an election, then maintain at least enough support to function in office.

And since they control, as the case might be, the nomination process and the funding of campaigns, they get what they want.