It just seems unconventional doesn't it.
I have pondered the same question, and have no answer. But - -with the bleeder down, it will have to be pressure fed from the bleeder back to the master cylinder. I made a pressure bleeder for my car and it worked great. Finally, I got a store bought pressure bleeder to prevent contamination with the aircraft fluid. It has an orifice on the outlet, the fluid will just trickle through!?? I hate spending good money for a product compromised for the brainless (litigants) that reduce the products' effectiveness in its intended use. It does not bleed as easily as my DIY unit. Now it will have to be "fixed".
Back to the original question, probably (purely a guess) attachment to automotive history (convention) and the fact they were bled from the master down, not the other way around. The bleeders leak trying to reverse flow it and overflow the reservoir too. Why? Because it is easy to push a bubble up or down in a small diameter line or hose, but in the caliper cavity, a large cavity, the bubble will come to the top, because the bubble floating velocity is higher than the fluid going down to a bleeder opening.
FWIW, Personally, I don't like pushing the most heat cycled and contaminated fluid back up through the system on a car, I like it going from the master down.