The quickest and most basic explanation for the tropes associated with the Stock Light-Novel Hero is to understand the differences between drawn media and written media.
Stoic, Ordinary High-School Student or Genius Bruiser Heroes
Light Novel protagonists tend to be more stoic or introverted than their Shonen cousins because the stories tend to be written in either first-person perspective or very close-third perspective. This means that the reader has access to every thought, realization and idea the protagonist has, requiring that they be rather observant to exposit on the world around them. And making them an Ordinary High-School Student provides a grounding for the reader to connect with the character and understand their thoughts and reactions. While it's not impossible to write an observant Idiot Hero, it's much more difficult—and unnecessary, thanks to the next point.Power Acquisition
Manga artists, especially those that work for known shonen publishers, are under pressure to draw impressive visuals and fight scenes that will amaze readers. On the other hand, Light Novels rely largely on limited amounts of text, and long, drawn-out fights are sure ways to eat up that text. In addition, reading about a fight doesn't have the same impact as seeing it, so it'll typically be brief and be constructed in a way that the writer can explain why The Hero won through exposition (such as unlocking a new ability).Harems Everywhere
The reliance on Harem Genre tropes is a related issue. As we said, illustrations in light novels are limited. So...if the creators can't rely on fights to impress the reader, then what works just as well (if not better)? Sex appeal! The occasional picture is perfect for showcasing one of the story's female characters in Fanservice moments, because it doesn't have to be followed up with anything. The picture stands by itself. There's also a bit of carryover from the Dating Sim and Visual Novel genres, which are also rather text-based and thus can transfer over well into light novels. Plus, a harem allows the writer to introduce new girls constantly, which adds variety.Extranormal Institute
The first subgenre that saturated light novel works was the Extranormal Institute/Magical Academy show which featured a protagonist entering some sort of elite school and rising to become one of its most famous and talented students. Examples include Maburaho, The Irregular at Magic High School, Absolute Duo, World Break: Aria of Curse for a Holy Swordsman, Undefeated Bahamut Chronicle, and many, many more. As theorized by YouTuber Mother's Basement
, the outbreak of these stories was caused by the popularity of Harry Potter both in the West and Japan. It made for a very easy form of Wish-Fulfillment where some otherwise Ordinary High-School Student (as noted above) can learn cool abilities that no ordinary school could teach, in addition to gaining the admiration of any girls that attended (especially if it happened to be an all-girls school). In the early 2010s, however, that genre began to slowly die out, largely to be replaced with...Summoned to Another World
The isekai plot existed before, such as in stories like Inuyasha, Vision of Escaflowne, .hack, and many other stories. However, these stories were more the Ur Examples of the trope and bear little resemblance to the sea of isekai plots in Light Novel stories. YouTuber Gigguk did the research and found that the trope took off in popularity due to the popularity of Sword Art Online (and, to a lesser extent Log Horizon). In fact, the protagonist Kirito became the bog standard definition of what a Stock Light-Novel Hero should be. Absurdly powerful, rather plain visually, and surrounded by gorgeous girls. SAO also codified the RPG Mechanics 'Verse trope in Light Novel works, which is another reason why the crossover with games is so prominent.