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Here's the problem - and it's not meant to stir any bad emotions on the topic of stable release...Just an opinion follows, nothing more...
We (VelocityWebHost) were actually going to release 2.0.13.1 with the Plesk autoinstaller and make it public doamin...why didn't we - too many bugs. Shortly thereafter, 2.0.13.2 comes out - but low and behold, we now have 2.2.0.
The last release we made a package for was 2.0.9...Ensim. We later upgraded all of our servers to PLESK dedicated.
The others (Fantastico, Ensim) stopped for either similar reasons or because they saw that the base install demand was low. For instance, Plesk is all about Mambo - well, we now know where mambo is going - joomla. Don't you love the forks/rebrands/whatever?
Now, we are waiting on 2.3.0/2.4.0 - when a "stable" single release is out, I think you will begin to see more auto-installer support. The industry (hosting) is saturated, but very selective in what they choose to do. It doesn't have to make a lot of sense, ultimately it's all about marketing and dollars.
I can assure you, we will have a XOOPS package for Plesk when there is ONE XOOPS package. I think XOOPS is on the right track, but progress has to be shown...
Now, to answer the question on what it takes. It depends on the installer, but you build the install with all of the features you want, you create a script (that sets up the settings), and package it up. Most want either zip or tar. You can usually get the spec straight from the hosting panel manufacturer. If XOOPS is successful with 2.4.0 (the ultimate goal), and packagers are made and distributed, touted as the highly secure, stable release of XOOPS, you will probably see a growth bump in useage. I just shake my head everyday I see loaders for PHPNuke - not my first choice, but "yet another CMS"