You may consider a relationship with another developer where they maintain the Android distro in exchange for some significant percentage of revenue.
1 - https://sensortower.com/blog/app-revenue-and-downloads-1h-20... - source needed, but speaking from experience at scale
There are a bunch of factors that probably cause this, but they're all mixed up and intertwined so it's hard to point at one and go "that's it, that's the reason". Selection bias of the user base (think: socio-economic status, and maybe even average technical know-how or comfort with software); iOS users use their devices way more than Android users, for all purposes (but, is that because of the previous thing? Maybe); it could have something to do with the ecosystems the two app stores have cultivated, causing different levels of trust among the two user bases when it comes time to spend money; it could have something to do with the quality of the experience of using the respective operating systems themselves.
Probably all of those contribute some amount. It's hard to say, though.
Do you have a repo or something for the Android app that curious people could look at?