Why did apple switch to zsh? Is it somehow better?

The version of bash shipped with macOS is VERY OLD (v3.2.57), but that version is still GPLv2 license and Apple is allowed to distribute it.

The current v5 bash is under the GPLv3 license that is poison to companies like Apple, so they cannot give you an up-to-date bash with bug fixes.

zsh is Not using a GPL license so Apple is free to include an up-to-date copy of zsh

If you want to continue using bash, you can.

If you want an up-to-date bash, as an end user you are allowed to install one. HomeBrew will do this for you <https://brew.sh/>, or you can build your own from sources.

I've used zsh for a long time, I forget why! :)

Probably because it is now under the GPL v3 license which is incompatible with Apple's policies.