I think it's a preemptive strike to handle supply/demand issues. They realized there was going to be a huge demand for this phone, and potentially not enough initial supply. I don't really "get" the invite system, but it seems to be working to keep people interested and hopeful of getting a phone in a short enough time.
In comparison, look at the Sony Xperia Z2. Delay after delay. Promises (or at least expectations) not being met. Possible issues with gaps compromising its waterproofness. It's an awesome phone, but people are losing interest because of these delays. Impatient people might have gone with a GS5 or M8 by now.