When I googled night hunter rengar skin this isn't what i expected

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This is true. I wonder if they've just run out of ideas for standalone anime?

The way I see it, it's a much better business plan to see something that's popular, and then make an adaptation of it. I mean Shiki as an example is an incredible anime, but it kinda just fell through the cracks... It was ripped from a book and so it didn't really draw a big audience that good exposure (Like previous Manga) would bring.

This started becoming very common after Attack on Titan, i've noticed. It happened before, but not as often. Now it's happening like 5 times each season. Let's see... Last season and this season (Don't know for sure) Parasyte, Re;Zero, One Punch Man, the new Berserk. Those are all that I can pick out, but as you can see it's a lot for two seasons, just 4 is a high number for Anime/Manga adaptations.

TLDR; It's safer financially to animate an Anime for an already existing fan-base, especially when it carries the potential to become basically globally known like AOT (Even if I personally believe AOT is severely over-rated.)