From what I could find, by searching on the net, is that most Light Novels are not written properly, both in the sentence-style, and in how characters and situations are constructed.
Here are a couple of quotes from a blog I found that might explain why adapting Light Novel to anime directly might be hard,
You see, these descriptions in LNs rarely happen from the point of
view of a detached third-person describing events, but are almost
always presented in the form of the protagonist narrating the events
they see. All these adjectives and adverbs are there to ensure we
don’t miss anything, and to tell us how the protagonist views the
They also show a certain uncertainty as to the quality of one’s
writing, to its efficacy in transmitting information without resorting
to this tool. If one trusts their writing, and if one trusts their
characters and situations to pass muster on their own, then you can
just present the scene and let people interpret the characters on
their own. Yes, some people might interpret things differently, but
that isn’t a bug, but a feature. Not so in light novels, we must at
all times know what the characters actually think, what is their take
on every little thing that occurs. The scenes aren’t allowed to
This is more than just slightly over-wrought floral descriptions,
however. It is more than just not trusting your audience to get what
you are going for (in the style of flashbacks). Another issue is that
since the author does his characterization that way, they aren’t doing
it in other ways – such as through the characters’ words and their
actions. Not just the other actions’, but the protagonist’s as well.
There is no need to “let actions speak for themselves” when you can
just narrate every thing you want to transmit to the audience.
There is something many light novels protagonists share, which is
related to the issue of flashbacks I mentioned previously – they
narrate. They are wry and cynical individuals who have lengthy
internal monologues. Most of what we know of them is through these
monologues. And here is where we reach the realm of adaptations. How
do you adapt such narrators? Either you have a “narrator track”, and
the character carries on monologues internally, such as Hachiman from
OreGairu or Kyon from The Melancholy of Haruhi Suzumiya, in which case
you reinforce their cynical and somewhat withdrawn personality, or you
simply cut it out.
And that’s where it gets messy. Since so much of the characterization,
especially of the main character is carried out through internal
monologues, if you cut them all out then the protagonist seems like an
empty shell. Yes, it’s the author’s fault, but when you adapt such a
character, whose actions and words don’t speak for themselves because
they never had to, you’re left with a “Too cool”, “wry”, and “slightly
withdrawn” character. The common complaints against shounen LN heroes.
It’s all true, but so are the cries of the LN readers who tell the
anime-critics that they’re missing on the true depth of the character,
which had never been carried over.
I think the thing is that it would be slightly more easier to adapt the anime from the manga (i.e LN->Manga->Anime) because most of the key frames would already be present and there would be a more clear understanding of the OG Creators idea or vision. I think it might also be slightly easier for the manga to adapt the narration aspect of LN's.
Having said all that, I think it is best to keep note that anime, especially the ones which adapt a manga or light novel, are mostly there to act as promotional or infomercial material for the OG content. And they sort of assume you’ve already consumed the original material.