Because you changed your shell to /bin/sh
If you then type “bash”, this shell is executed - and you get a different prompt.
You don’t need sudo for that, btw. (only when you want to change it for another user than yourself)
If you want to use bash, change the shell to bash instead of sh …
chsh -s /bin/bash
You don’t want to use /bin/sh as your default shell - use bash or zsh.