The rules of each America's Cup are decided by the previous winner and challenger. For this one they also added a bunch of America's Cup Series matches using smaller boats (that are still huge - the one hanging from the ceiling in Cal Academy of Sciences is pretty cool) and extending the regatta. Basically it's all for marketing and advertising money.
Maybe it will change back next time when they race down under...
I don't follow it, but I am well aware that it is a VERY expensive undertaking, so I can fully understand why the sponsors who pony up the cash want it to be a multi-phase/several day event.