I used to sell books and work closely with publishers. Just like any corporatised industry, it's all about the money.

First tying an author into a multi-book contract means you pay less for the books. If you are bidding on individual titles, you pay more as the highest bidder gets the rights. This also benefits the author with job security.

Repeat publicity. Single title comes out, everyone raves about it for a few months, then it goes into obscurity. Series offer the chance to repeatedly plug a title with each new instalment.

Why let you buy one book when we can make you buy it three times? This is he DLC of the publishing industry.

Longer lasting sustainable contracts with booksellers who want reliable tittles to keep drawing in customers.

Trilogies tend to be fairly crap but a majority of readers tend to also be fairly lazy. Books like Brave New World are deep, convoluted, and require a robust sense of the world to truly understand the meaning and message. Most trilogies I have read tend to focus on novelty as opposed to message.

Publishers recognise that messages can be very dangerous if they are not the 'right' message to fit the audience. Trilogies like Divergent (now with a spinoff) are anti-science, anti-government, and pro-religion, and sold very well across the US but did relatively horrible in the UK. The His Dark Materials trilogy are anti- established religion and very pro-science, so it sold well in the UK but was all but banned in the US. Trilogies with titles like these draw news and attention and as the saying goes, 'all news is good news'

I tied to keep this as a very superficial account of what I saw in the publishing industry in regards to trilogies. There are much deeper economic reasons, but while I understood them at the time, I don't really know how to explain it without writing a novel. And as I am typing this on a phone, i don't want to be here all day! Also pardon grammar and spelling, autocorrect may have kicked In along he way.