Why is Bloodmoon Akali drastically different than other bloodmoon skins style wise?

Because she's an old skin that hasn't been updated for the new themes of the skin line.

ut has to do with the old lore, she was a human pretending to be a demon, not sure if its cannon anymore

In the original lore, the Blood Moon skins represented heroes of Ionia acting out a play. Shen played a demon and Akali played the hero.

It's a very, very, very old skin. I'm sure it'll be revamp to higher quality with the recent Bloodmoon skins whenever she gets a VU.

Release date will answer all your questions. Back then, when Riot (and pretty much any game developing company) simply didn't have the tech to do anything with her. Akali's kit is also hard to work on in terms of effect, HH Akali, which is a semi-legendary skin still feels lackluster because there's just not much to do with her kit.. Not according to the coding capabilities and ideas they had back then. If it was released now, she's probably throw a dark-red smoke canister with BM themed visuals for her W and probably just a bunch of BM effects spread all over the kit. I can see how it would work, I think this skin deserves a second chance to shine (as in a remake of it.. After Riot's done with all the huge projects like Eve, Galio, Morde, Urgot and the tanks).

And if it's a candidate for a remake, then it's behind other skins like PF Ezreal in line.

Also, I think it's the one and only BM skin that exclusively, does not represent a demon, unlike the rest.