Mostly because it's so easy to add a category.
Another thing is that
one page typically belongs to several categories that can have
different access mode. I think it's possible tom implement
per-category access but there will be many interesting nuances.
That's correct... But on the other hand, categories are the main
structuring tool in MediaWiki. And people want to apply rights based on
their existing page structure. It's the problem of flat structuring -
access rights are simpler and more evident in hierarchies.
The funniest thing I can think of is when I'm
editing the page and
add
the category to which I don't have the view permissions. I click Save
and my article dissapears from the view :) . Of course it's solvable,
but I suspect there is a lot more similar cases that will require
some
additional efforts.
In both HaloACL and IntraACL category isn't like a mandate - to the
contrary, it's like a grant - rights from different categories are added
to each one.
The case in which the article disappears from your view after adjusting
access rights will always persist (or the extension should check for the
loss on each edit), and one needs to call admins in that case, because
MediaWiki has no "article owner" idea (which is definitely good for
encyclopedia, just like the flat structuring). Either the extension
should implement "article owners"...