I think it's more to do with manufacturing and inventory management. With the invitation system they can smooth-out demand and prevent peaks until manufacturing is well underway.

Of course one of the ways in which it smooths demand is to turn people away; I was interested in a Meizu Ubuntu phone until I learned I had to 'compete' for the privilege of giving them my money.

Same with the original OPO, by the time I won an invitation ( after waiting several months ) I had gone and bought an LG G3.

The opposite. To little supply for the demand. They used the invites to "throttle" the purchase rate to be able to keep up. They probably thought an invite system was a better aproach than a weekly free-for-all aka. SuiciDDOS against the webshop. Hopefully, this time supply will be significantly higher, meaning the invite system will be removed as soon as they know they can keep up with demand.