Probably because of the PoE. That discards SFP+, and makes difficult PoE over copper, as you'd probably need 802.3bt PoE++ (that probably most of the Unifi devices aren't compatible with), or a very short cable to avoid interference.

10Gb interfaces also tend to run quite hot and be a bit power hungry.

This is a device that needs to be in a location with good 5G reception, so it makes sense to be PoE powered so you can put it near a window or in the location that gets the best reception, and only run a long ethernet cable. And, although I don't like it too much, 2.5G or 5G NBASE-T is the nearest thing that covers 5G speeds.

The 2Gb downlink speed is the 5G downlink, the max for the whole 5G connection, so 2.5Gb ethernet is enough for that.