Why do only two candidates advance to the General Election?


Because a single choice vote is only equal if there are two candidates in the race: otherwise the more similar candidates split votes, which disadvantages voters who like more candidates. Since voters are aware of this, today's current elections are effectively a top two already - one candidate each from the major parties. Although minor party and independent candidates can compete in today's General Election directly, they are unable to compete effectively in no small part because of the spoiler effect and the “don't waste your vote” phenomenon.

From the candidate’s perspective, this is also a benefit. Each candidate has one clear opponent for the General Election against whom to compete and differentiate, versus an uncertain field of disproportionately aligned opponents.