Because it kills the economy and forward momentum of a society, puts a huge burden on public health and other services, creates a elderly society with less "new blood" innovation in ideas and politics that stifles the (fewer) young people, and beyond some point it's not even reversible without the country falling many places in GDP and standards of living.
Population decline in general isn't necessarily bad. Demographics highly skewed towards old people are problematic though, and if nothing is killing off old people that's what you get when birth rates go down.