Like most human related things there is no single cause.

I can't comment on Iceland.

However, South Korea has one of the most crushing work cultures in the world, with insane work hours. Most people are not even in a relationship yet alone with kids.

In Portugal you have a huge problem of disposable income. Median salaries are insanely low and stagnant when adjusted to COL. To the point that by now most former 'Iron Curtain' EU members have surpassed Portugal in workers income both adjusted and absolute.