You don't really ?! Its a standard with this economical mode - per month donations. You put out content until you hook enough people to support you. Then lay back and do just enough content so the pay check doesn't run dry. If the support start to dry up you can bet your ass the updates will come out faster. 90 % devs are like this. And its natural why put more work when you can put less when the money are still the same. In this game's case the is not reduction in donations , hence no hurry to do work.
Standard, if you pay someone for a result, they will do it directly and work for it. If you pay people a small amount every month, they will relax...
Having said that, often also the case, dev has a nice idea, puts it out, and then doesn't know how to continue or finish the story. We all have nice ideas, but try to write it down, then you see you are missing a lot. And if you don't, you got talent, and can make something from it.
We can't say for sure with this one, but it's the usual, sickness, getting a kid, burn out, depression, and other personal problems, and such. Delaying development and such. I got a number of them in my own life. So I can't blame them. Losing your mojo doesn't help either. Sometimes you just sit back doing nothing, in the hope you get some inspiration, but it doesn't come, and before you know it, a year is past. Or in some cases, 10 years passed...
We can only hope it will continue, and unfortunately that takes time...