My initial thought was the same - USB 3.0 is the same as USB 2.0 functionally, it's just faster (as far as end users are concerned). So it doesn't make sense not to include it. Bluray on the other hand is a marketing decision, Apple wants to move away from optical media.
But then, that doesn't really work, under that logic. Functionally, a Bluray disc is exactly the same as a DVD, only with more capacity....
if i remember correctly usb3 has different system to connect fast devices and usb2.0 support from the normal port
and it can give twice the power than 2.0 could