Why did Kurt start writing in a different style between Bleach and Nevermind?

I’ve often wondered what sparked the departure in style between the two albums. Needless to say the songwriting took on a poppier element, but he also start writing more songs in standard tuning, and also did the loud quiet loud thing. Bleach riffs were more single string orientated rather than power chords. Is there something specific that made this change happen?

Edit: you’re all a bunch of insightful fuckers, thank you so much for the info.