Until you realize where their market is for this. There are some places that require a 4 stroke motor. The 600 ACE is probably the lightest and cheapest 4 stroke on the market.
You're right, why would anyone get a 600 ACE when you can double the HP for roughly $1600 US and still get pretty close to the same fuel economy.
Or staying in the Skidoo line, the 550F puts out 65 HP and is less expensive.
It all goes back to "4 stroke required"