I don't know. It's a thing in many countries. I assume that many either don't know who to vote for or just don't care about voting yet. Maybe a combination.

Last time (in 2023) only 45,5% of norwegians aged 20-24 voted. For those aged 25-44 it was 52,5%. After that it grows rapidly.

I cared less about politics as a teenager which made learning more about each party less interesting. I've been gradually caring more as I get older. I assume many experience this to some degree.