Sorry for always plugging myself, but I've reviewed about 23 adaptations (some looser than others) so far, and I have some ideas:
1) Directors don't actually care about Lovecraft and want to attach his popularity to mainstream horror without interacting with the author's works in any meaningful way. See: From Beyond, The Dunwich Horror, The Haunted Palace, The Unnamable, Cast a Deadly Spell, The Resurrected, and Call Girl of Cthulhu. Sometimes it works enjoyably enough (The Unnamable & The Haunted Palace are, I think, quite delightful and solid movies on their own right thanks to a focus on character writing). Usually, though, it's hackneyed.
2) Directors want to add their own "twist" on it, but usually that twist is a bland reflection of the times. See: Dagon and most of Stewart Gordon's stuff. He wants to be "edgy" and add tons of sex, but it's actually misogynistically exploitative, a reflection of the "dark and gritty" sexuality mixed with horror schlock to pander to a fanbase, pretending to have some affinity for HPL but really only having an affinity for shallow sexual titillation.
3) A complete misunderstanding of what makes Lovecraft effective. See: The Whisperer in Darkness, most Gordon movies to an extent. They think Lovecraft is great because he had cool monsters with an undercurrent of mythology, rather than a distinct philosophical perspective with a root of fear in the unknown; but most importantly, they misunderstand Lovecraft's aesthetic. I don't mind updating an aesthetic, which many attempt to do, but it needs to be updated interestingly.
4) A lack of real talent to back up interesting impulses. See: Cthulhu, Out of Mind, and various short films. They have a rare view and try to take Lovecraft and push the boundaries of what he did while understanding what boundaries they're pushing. However, the budget can't match the ideas, the direction is either overwrought or a bit uninspired, the writing totally doesn't click (usually due to lack of realism or lifelessness), and the filmmakers can't figure out how to make it all work.
Basically, there's not enough interest from big talents. Those who are interested seem to have a surface-level understanding and interest in Lovecraft, and there's just not much room for a nuanced adaptation.