Now that CDU is in opposition they spew out even more right wing rethoric in the hopes of gaining voters from AfD. But that also legitimises that view.
FDP is severely hindering the government, thus anything good it can do has less impact.
Now that the government is trying to rectify some of the problems CDU (and SPD with them) caused when they were in the government those problems become more apparent. Angela Merkel was great at deflecting all criticism and progress.
SPD hasn’t really been a social party for decades. That scares people away that actually want a social party. Die Linke isn’t feasible either, they’re deep in Putin’s arse and hate immigrants as much as the right.
I think it’s due to several factors.