Because everyone owns one and wouldn't need to buy another?
Your assertion is that Blackstar profitability on this product is mostly tied to the secondary sales of the 6.5V power supply (the power supply is not included in the base product, but apparently a set of decent batteries are included which, by the way, would cost about the same as a typical China-made power supply at volume)? Seems unlikely. Particularly when this is one of the most compelling reasons AGAINST buying it in the first place. And it is in such contrast to the rest of the product design which is otherwise brilliant.
The power supply is included in the 2 speaker kit and perhaps it was intended as a further motivation to buy the upgraded (and certainly more profitable) kit. But why not just make it 9V then as well and remove the biggest "against" in a prospective purchaser's eyes?
Roland did something similar with the 2nd generation Micro Cube. The first gen used an external power of 9V (and 6x1.5V = 9V of batteries). The second gen (GX) for some bizarre reason switched to 12V external while still 9V on the battery side. At least in this case, there is a chance Roland was trying to provide louder operation when plugged in, which has no chance of being the reason for the Fly at 6.5V external.
The 6.5V decision seems so blatantly stupid that I have to imagine I am missing something subtly brilliant.