Because alarm_occupancy is not a valid Homey capability…
Ditch the Athom HA app 
The following is only possible with the very cool Home Assistant Community app:
(installable side-by-side with the Athom HA app)
It most probably gets it right when you add a new HA sensor. When it doesn’t, no worries;
Just edit the device in Homey, and add the motion capability again (maybe it has to be removed prior to adding)
Make sure to use alarm_motion as Homey capability.
Xmpl:
