One click installs are maintenance intensive for the provider, and often bring many complaints from users that it didn't do it the way they wanted it done (or with options selected they wanted etc)
Then there is the upgrade cycle. user: "you broke my site, my xthingee plugin isn't compatible with the new version yet, I don't know what to do now. Put it back the way it was, NOW! Put it back, my sites losing money every minute that it down." (I left the "please" out of the example on purpose.)
On the other hand, installing web apps is pretty simple. Do it twice and you're good at it, do it five times and you've mastered it.... very very easy, generally: wget a file, unzip it, run it. Done.