Because transpacific flights from US East Coast cities are close to unbookable in the fare classes RTW tickets need. Sydney to JFK direct came back at 0% bookability across 235 tested samples. Sydney to LAX is 80%. Fly domestically to LAX or SFO first, then cross the Pacific.
Because the direct flights from the East Coast exist but do not book. Round-the-world fares require specific fare classes, and transpacific routes out of East Coast cities almost never have inventory in them. Sydney to JFK direct came back at 0% bookability across 235 tested samples. Sydney to Los Angeles came back at 80%.
That is not a marginal difference. It is the difference between an itinerary that tickets and one that falls apart at the pricing stage.
The practical rule: it does not matter whether you live in New York or Miami. Fly domestically to LAX or SFO, then cross the Pacific from there. Vancouver (YVR) is a workable third option, with reasonable availability on Air Canada and Cathay Pacific, and it suits travelers in the Pacific Northwest or western Canada. LAX and SFO still carry the deepest availability.