I would have to guess that the mesh signal strength of the USB-based ZigBee radio that Hubitat chose to use with their hub is perhaps not as strong as the ZigBee radio inside ST's v2 hub. I have read a couple of user reports which seem to indicate this. There are a lot of factors that affect signal strength, of course, so it's not necessarily just as simple as that.
Certainly the general advice here would be to consider adding mains-powered ZigBee devices that act as routers (repeaters) such as the Iris SmartPlug mentioned by @Tony, some posts above. However on SmartThings forum, there have been quite a few reports of repeaters not working well with Xiaomi devices. That still needs to be tested for the Hubitat, and we've only just possibly seen an improvement in the devices staying connected as of yesterday - so some time and more testing is needed to be surer about it all.
