A friend asked, why does Ngozi do Kickstarters for a year after it's complete?
Because printing books is expensive. She gets her monthly income off Patreon, but if she wants the books done well, she has to go to the printer with money in hand and order a few thousand copies. If you pay to have your book printed but don't sell enough of the resulting product, you don't earn any money. (Getting a publishing contract is essentially having a company agree to provide the printing costs and accept the loss if they misjudged and printed more books than actually sell. I mean publishers also offer editing, publicity, and many other services, but printing is the mammoth line item)
First Second have offered her a publishing contract, but it has a lot of restrictions and restraints. They want the books formatted and produced a certain way that she probably gets very little control over (authors usually don't even get to say, "I don't like that cover, it's ugly, change it"). If volume one (years 1&2) doesn't sell well, they may choose not to print volume two at all. If volume two does get printed, it won't be until after year 4 is finished--and because they want time to schedule it at the right time in the book release calendar, get it reviewed by the right people, and get enough publicity up and running for it, it might not be released until months (a year is not unrealistic) after they have the completed story in hand.
So Kickstarter is Ngozi's chance to have the book printed now, for the fans she has, the way she likes.