The short answer is timing.
About two years ago I met with a major publisher who was interested in my first book, which became The Lost City of Zarahemla. I was told the production timeline was 18-24 months.
IOW, had I gone with that publisher, I'd just now be coming out with my first book. Instead, in the same amount of time, the publisher I chose has released 7 of my books with two more to go this month. Not that speed is the top priority, but it's an important factor because so much is happening and time is short.
Another reason is that in my experience, the academic world has one way of viewing the issues I'm studying and writing about. I don't share their viewpoint, and I don't think they're open to alternatives to what they already think. From what I've observed, at least in LDS fields, peer-review is peer-approval. There is a high level of groupthink, more than I've seen in other fields, and I think it's at least partly attributable to a siege mentality; i.e., many LDS people feel like it's "us against the world." As I've shown in several instances, people are more comfortable confirming their biases than in challenging their assumptions and taking a fresh look at the evidence.
That said, I'm fine with people believing whatever they want. I don't write these blogs or the books to persuade anyone, but just to explain how I see the issues and how I assess the evidence.
I like to get feedback and I've sought feedback before I've published everything that's out there. Feedback is essential to improve the research and writing. As I've mentioned many times, the critics have been very helpful. Usually, they complain because I don't see things the way they do, but sometimes they give me good ideas and sometimes they point out mistakes, which I try to correct immediately or as soon as possible. (I'd rather give and receive criticism privately before either side publishes, but those who disagree with me refuse to give or receive private criticism. In fact, they publish criticism of my work but refuse to publish my responses. That's another major reason why I blog.)
Another reason: because I'm not in academia, I don't have an obligation to "publish or perish."
Another reason: my readers frequently contact me with additional insights and material that I can incorporate in future printings and editions, as well as blogs. I figure the more these ideas are disseminated throughout LDS society, the more additional data people will send to me or share with others. Eventually some of these ideas might make it into mainstream LDS scholarly circles, and I'm always in favor of that, but I'm more interested in ordinary members of the Church who are smart and well-informed and don't need a scholar to tell them what to think.
There are parallels in the non-LDS world. For example, a climate scientist whose work I respect recently resigned from her tenured position at a well-known university. She explained: