I don't have the bookmarks bar visible by default, so bookmarklets are basically useless to me.
Good question. I use qutebrowser, which can run the equivalent of bookmarklets, but not plugins. One of the reasons I like it is that I can map any keys to invoke any script, including, but not limited to, javascript snippets.
One possible explanation: the individual creating a plugin instead of a bookmarklet is unaware of bookmarklet functionality.
There are likely many other plausible explanations.
Bookmarklets can't circumvent CSP policies (or at least couldn't always, not sure if this has changed), extensions can.