It's not even entirely up to them. They made changes to their original ZigBee software stack due to community feedback that, as they thought, might help Xiaomi devices stay connected. Beyond that, there's little within their control. Xiaomi devices use a proprietary/custom ZigBee profile, though it happens to be close enough to HA 1.2 that HA 1.2 controllers like Hubitat are usually able to pair to and communicate with them. What messages they send has basically been reverse-engineered by dedicated community driver writers (mostly just one lately) since Xiaomi only officially supports these devices on their hub/gateway.
The best Hubitat could really do is include the drivers natively, but I think that would be a bad idea since people might imply that as official support, whereas the devices are problematic if you don't have the right kind of ZigBee routers (Hubitat can't change those devices' behaviors--I'd say that's the biggest remaining problem now, and it's entirely out of their control). Adding a custom driver isn't that hard, and it's probably a good way to keep people who aren't good at reading directions away from trying them and causing more headaches for themselves.
Also, I have seen another person or two say they've had better luck on ST with Xiaomi even though "everything is the same." It is, of course, not, since the hub (and its radio) are different. The ZigBee radio on the Hubitat stick is likely (guessing based on community experience) not quite as powerful as the one built-in to ST, or at the very least it's almost certainly oriented differently. I actually have had better luck on Hubitat since now I know what repeaters work well--my guess is many people could pair them directly to the hub before and never had to worry about this but now do since the end devices might find one of those routers instead.