Because the actual plan was to build-up solar and wind, then phase out nuclear and coal.

But the conservatives intentionally sabotaged solar power and wind (see here and here) and also blocked grid imporvements and extensions to keep their beloved coal alive. After more than a decade we should long be past the point to not need coal anymore (Just look at the graphs and extrapolate the amount of solar and wind without their de facto destruction of the solar (2012) and wind (2016) industry via overregulation), it's still a big chunk of the produced energy.

Nuclear was simply phase out because the existing capacities were rediculous low (~5% of the production top), the shutdown was already decided and planned for years and keeping them few reactors alive would have costed rediculous amounts compared to their value. And completely restarting nuclear basically from scratch makes zero sense today, when you won't need it in 15 years anymore.

This is pure and simple the result of corrupt conservatives pushing coal and their propaganda (killing 100k jobs in solar production to protect 10k coal miners for example). And instead everyone now eats up their propaganda again and blames the current government, not only for the problems but also for a nuclear pahse out that was actually decided and prepared since a decade ago.

why didn’t they decide to “phase out coal” instead?

23 years ago, under Schröder they wanted to phase out both and massively subsidize renewables.
Later those plans were largely axed under Merkel.

The reason why coal has a strong standing in Germany is that it is one of the few natural resources the country has.
It has long been a staple in certain political circles to justify coal subsidies by pointing to the many jobs tied to coal mining.
Nevermind that they had no problem throwing jobs in the solar industry under the bus when they cut subsidies for that.