My guess? Not getting the word out there enough that the Patreon exists. Hell, the 1st post lists the Patreon under "Mercenary Mage" instead of as "Patreon." For those people that HAVE played and haven't supported? I'd guess it's the between scenes UI. It's really clunky, doesn't give a sense of progression, muddles the timeline of events, and detracts from the sense of your being IN the world. That's all fixable, but would take a deal of effort. I'm sure if that was addressed more people would throw in support, but I may be wrong.
Don't get me wrong, I like the game, but I think that's what's keeping support low.