Why is that?

When invoked with -c, bash runs as a non-interactive shell. The default interactive shell startup files (/etc/bash.bashrc, ~/.bashrc) are not read. Even if you force ~/.bashrc to be read, its body is likely not executed - the default Ubuntu one (copied from /etc/skel during account creation) for example includes the lines

# If not running interactively, don't do anything

case $- in

    *i*) ;;

      *) return;;

esac

which bypasses everything following.

You could force an interactive shell using bash -ic, however that may cause other issues depending on what else is in your .bashrc.