Short answer: New builds get $10,000 because the government wants to encourage construction. Existing homes get $5,000.
The government wants more houses built. So they pay $10,000 for new builds but only $5,000 for existing homes. It is their way of encouraging construction. If you buy off the plans or build new, you get double the grant.
Years In Kiwi Saver: 5
Existing Home Grant: $5,000
New Build Grant: $10,000
Difference: +$5,000 for new build