Putting my teacher-librarian hat on here:
As children in the 60s, my brother and I both read his copy of "Star Trek: Mission to Horatius" by Mack Reynolds. We enjoyed it, and my brother's book secretly made its way into my growing ST collection in 1980, but the book's content wasn't anything Earth-shattering. At least not for us, although it may have piqued our curiosity for TAS when we found that in b/w on Saturday mornings in the 70s.
Having met many members of ST's "first fandom", those who were watching ST as kids and who also craved reading about ST were devouring the James Blish (TOS) and Alan Dean Foster (Filmation's TAS) episode adaptations. The selection of ADF as writer of the 2009 ST novelization was met with much enthusiasm from those ST fans who'd learned to read with "ST Logs" 1-10.
Although I have the delightful Lawrence Weinberg "ST III Storybook" in my collection, and have bought additional copies for all three school libraries I have taught in over the years, it never gets borrowed. Likewise, child interest in the Pocket/Minstrel YA books for TNG, DS9 and VOY has been minimal compared to the war SW kids devour SW YA books.
If you look back at other seemingly YA ST books, they are also more appealing to adults (as collectibles) than to kids (as reading material). "ST II Biographies", the matching short story collections and choose-your-own-adventures for both ST II and ST III, the ST IV YA ("Young Adult") novelisation, the TNG movie YA adaptations... how many sold to/for kids, and how many went straight into adults' collections? Ditto the old TOS and TMP pop-up books, and Daniel Cohen's "The Monsters of Star Trek".
I do know some kids who collect all the "Star Wars" YA books, but (like "Goosebumps" books and "Ben-10" books), they collect them as they would gum cards, and they don't necessarily use the books as reading matter.
Both SW and ST can appeal to young kids, but the ones inspired by ST seem to have no problem becoming consumers of the regular ST novels, thus skipping the YA stage completely.