For the same reason as raising a billion dollars of series A funding: it's not about what you need it's about what people will give you.
Despite all the defensive language, kickstarters are really pre-sales. 90k sales for a book is very respectable but not overwhelming. It means he's found a great market for this book.
The kickstarter was for 2K, which seems like a reasonable "minimum sales" target to start a printing run. The fact that 90K was raised seems to prove there is demand for that kind of book.
Always good to look at "How To" books with a healthy dose of skepticism. Probably more relevant to the "get rich quick" type books, because if they got rich quick, why would they be selling a book to give away their secrets?
Although I find things like farming and agriculture and outdoors to have a less capitalistic shade and more of a genuine "i want to help people achieve their goals and not die" shade.
I'll be very upfront: homesteading is a money losing proposition. It's a consumption good - it costs more than any (monetary) profit it brings.
And my writing about it is ALSO money losing. I'm a coder. When I write and sell novels, the opportunity cost is huge - at an hourly rate, I lose about 90% of what I could otherwise be making coding.
Writing about homesteading is much better - I only lose about 50% of what I could otherwise be making.
I write because (a) I am more driven to create and share my ideas with people than I am to make a marginal dollar, (b) I am ideologically in favor of people moving to the countryside and living a different lifestyle, so writing is an ideological / political choice.
Interviewing three people is also apparently worth 5K.
And the book's photos are black & white ...