Man, I hate the idea that you have to write differently for a teenage audience. Teenagers aren't stupid. They're perfectly capable of reading on an adult level, and most resent these kinds of books for the simplified nature. I despised it when I was a teenager, and I'm not any fonder of the idea now, honestly.
Okay, putting my school librarian hat on here:
Some kids are " perfectly capable of reading on an adult level", but
lots of kids - and
way too many adults - are not. If
you read any of the previous ST YA novels, and didn't like what you were reading, perhaps you simply weren't of the same reading age as the kids those books are designed for? That's why it looked to you like they'd been dumbed down.
YA novels are a misnomer, but they're called "Young Adult" novels because they are aimed at
kids who are still quite a way off being "young adults". That's all part of the marketing strategy. Kids with lower reading ages than their chronological age, lower than all of their peers, and who are not
ever reading for pleasure.
By using a more controlled vocabulary, shorter chapters, larger fonts, colour plates, b/w illustrations, appealing covers, young protagonists of slightly higher age than the intended readers, and highly-covetable media tie-in titles, otherwise reluctant readers can often be cajoled/coerced/tempted into "having a go". YA books are aimed at kids who do not usually find reading to be pleasurable. But... often
motivation for reading is all that was needed to kick a kid's ability to strive for higher reading ages. They already had the basics (word attack skills), after all, but felt no compulsion to use them because reading was a yukky thing the brainy kids do.
Other franchises has books and other tie-ins that appeal to younger age groups--Lord knows Star Wars has been doing it since day one--and Trek actually used to do that itself with storybooks before it suddenly kicked kids to the curb at some point...
If YA novels do their job properly, the kids who do "switch on" to reading with them won't need YA titles forever. If they are really engaging with the topic, they'll often do a leap of faith to related adult books on the same themes/characters/genres.
Marketing/sales will always have an influence here, of course. As one of the editors explained a few years ago, if there's already a sizable collection of YA books on a certain topic, the publishers will often move on to a new funkier, more popular topic-of-today, because kids pass through the "need to read YA titles" phase quite quickly and they are eager to move up to adult novels in the same genre. You'll often see media tie-in YA titles come out in blocks of four, maybe followed by another block of four - and then that might be it, because the
next YA demographic cohort is already champing at the bit for the next "in" thing, not last year's thing. Meanwhile the old YA demographic's cohort members are either reading adult books (the job done) or back to more physical things (sport, computer games, dating).
Writing correctly for the kids/teens market ain't easy.
You bet. "Young adults" may not be confident readers but they can spot a phony author a mile away!