Pretty much the biggest problem with supporting Habits was a lack of time to write the code. 
I was working on the Beeminder-Habitica integration but I was also massively over-committed on other things and still havenāt really recovered from the burnout.
There are some technical limitations that would make Habits harder to track in Beeminder than Dailies and To-Dos. I wrote something about it at Habitica: How it Works; How Beeminder's Integration Works and meant to write more butā¦ yeahā¦ time.
Habitica does have webhooks, including for when a task is āscoredā (completed or plussed/minused), so setting up a webhook in Habitica that populates a Beeminder goal via a script or via IFTTT might be one option to work around the current lack of an official integration.