They are not in the list of compatible devices because they are not officially compatible. 
That being said, many people have been able to make them work, usually with success. The first post in this thread summarizes what you need to know: Xiaomi & Aqara Devices - Pairing & Keeping them connected
To summarize, you need custom drivers (available in a separate thread linked to from that one). You also need to be careful how you build your Zigbee network; only a few repeaters are known to behave well with Xiaomi devices, while others are likely to make them fall off the network. The latter seems to be a common problem for many people. That being said, in most cases Hubitat should still be able to discover the sensors, so if that's not happening, they likely weren't reset or properly put into pairing mode. Consult the "Basic Pairing Instructions" section in that thread for information on how.
If you want to avoid these headaches, sticking to officially compatible devices is the best solution, but many more will also work without problem (any nearly standard Zigbee or Z-Wave device should work; the current Xiaomi devices are close but not quite standard).
