I don't think it's a technical reason, just a design reason. That's not what
user controls are for.

User controls are meant to be a simple way to build a composite control
without writing any code. They're not meant to be general purpose.

Now, at some time in the future, Microsoft could introduce a visual
composite control editor that might take the place of user controls for many
purposes, but user controls are meant to be more of a replacement for
server-side includes in ASP than they are meant to be real custom server
controls.

That said, it's not that difficult to write a composite control which is the
equivalent of a given user control. It takes longer and requires more
knowledge of controls, but it's not very difficult.

John

That's what Custom controls are for.  Custom controls have great
reusability, but they're kinda hard to make.
User controls are only reusable within a single project but they are easy to
make.
Pick your poison.

Here's more info:
http://SteveOrr.net/faq/UserCustom.aspx

Look at ScotsGu's tutorial on "Building Re-Usable ASP.NET User Control
and Page Libraries with VS 2006"
(http://weblogs.aps.net/scottgu/search.aspx?q=user+control&p=1)

I also have seen an approach of embedding the source of .ascx in a
resource. This resource gets compiled on the fly.