I published my homebrew, but now I want to make a change to it. Why can't I delete/edit the published version?

Once homebrew has been publicly published, it may be added by players and in use by them. The player who has your homebrew may like it how it is and not want the change(s) that you are making. You should create a new version of the homebrew, with your changes and then submit that, then others have the choice to change to your new version.