Unless someone here happens to work on the Windows Shell team, I doubt that you’re going to get an answer that really addresses the technical limitations and how they affect the design choice. But I’ll try…
My guess is that there isn’t any technical limitation, or at least there isn’t one now. The real reason is presumably that no one has ever taken the time to sit down and update the code, the design, and the spec to lift this limitation. Features aren’t implemented by default, and just because the computing environment has changed in the last few years doesn’t mean that someone sat down and rewrote Windows to take full advantage of all those changes.
You should also consider that is more than likely a conscious design choice, rather than an imposed limitation. Raymond Chen (who actually does work on the shell team) published a blog entry responding to the uproar about Windows 7 removing the “sharing hand” overlay. He makes a compelling argument that the icon overlay is really not a desirable way of showing information (above and beyond the fact that the system is limited to 15) [emphasis added]:
Generally speaking, overlays are not a
good way of presenting information
because there can be only one overlay
per icon, and there is a limit of 15
overlays per ImageList. If there are
two or more overlays which apply to an
item, then one will win and the others
will lose, at which point the value of
the overlay as a way of determining
what properties apply to an item
diminishes since the only way to be
sure that a property is missing is
when you see no overlay at all. (If
you see some other overlay, you can’t
tell whether it’s because your
property is missing or because that
other overlay is showing instead of
yours.)
It seems reasonable to me that the extra clutter added to the shell is simply not worth it in the majority of real-world cases. The Windows Shell team obviously reached the same conclusion and cut the “sharing hand” overlay. Raymond’s direct explanation:
Given the changes in how people use
computers, sharing information is
becoming more and more of the default
state. When you set up a HomeGroup,
pretty much everything is going to be
shared. To remove the visual clutter,
the information was moved to the
Details pane.
And, I know you specifically asked not to mention performance, but Windows really does try to keep you from shooting yourself in the foot. Users demand responsiveness in the shell, and overlay icons can interfere with this. As further evidence that they are not the priority, another blog post by the same Raymond Chen chastises:
Another example of applications having
a selfish view of performance came
from a company developing an icon
overlay handler. The shell treats
overlay computation as a low-priority
item, since it is more important to
get icons on the screen so the user
can start doing whatever it is they
wanted to be doing. The decorations
can come later. This company wanted to
know if there was a way they could
improve their performance and get
their overlay onto the screen even
before the icon shows up,
demonstrating a phenomenally selfish
interpretation of “performance”.