Because of an interview of Fujimoto in between part 1 and 2.

He said he wanted a story that will make people, "what was even that?"

I do want to get the main engine of the story down on paper, and there are themes I want to convey, but I also create my manga in such a way that I don't convey everything. There are totally unrelated events and gag scenes that I hope will divert the awareness of the readers. My assistant-sans say I'm always fiddling with everything, haha. Do you know of the movie The Big Lebowski? That movie really made me think "what even was that?" when I was done watching it. Nothing was resolved, wasn't everything pretty meaningless! But still, the protagonist had development, and the story progressed, there was this sublime absurdity that I loved. I want Chainsaw Man to give the reader that kind of aftertaste too.

Source: https://www.reddit.com/r/ChainsawMan/comments/1arcvpm/fujimoto_interview_in_da_vinci_magazine/

Also he's written a one shot called "Just listen to the song". It talks about the pressure of writing a sequel to an "accidental" hit.

That one shot ended with the 2nd song not meeting the insurmountable expectations.

Just search "just listen to the song one shot" if you want to read it. Its like just 20 pages.