A big, bright screen forced a big battery.
I don't know what the battery usage on a HUD screen is, but I suspect a lot less - they don't require backlighting for a start.
That cuts down on the battery requirements, which cuts down on the size & weight (Obviously this is speculation: it wouldn't be a huge surprise if you had to carry a battery pack wired to the glasses somehow)
Secondly, the glasses themselves need to do a lot of fast, low-latency image processing. It isn't at all clear that bluetoothis an appropriate low-latency, high bandwidth connection mechanism for this.
Having said that, I suspect you'll be able to connect to your phone via bluetooth if you want - but the primary processing will be done on the glasses.