Allow a prefix in the vocab attribute?

As far as I can tell from the current spec, @vocab is expected to be a URI.
If a prefix was allowed, it would allow for shorter RDFa snippets when
combined with the RDFa core default profile [1].

The following snippet taken from the RDFa 1.1 spec:

<div vocab="http://xmlns.com/foaf/0.1/" about="#me">
   My name is <span property="name">John Doe</span> and my blog is called
   <a rel="homepage" href="http://example.org/blog/">Understanding
Semantics</a>.
</div>

could be rewritten:

<div vocab="foaf" about="#me">
   My name is <span property="name">John Doe</span> and my blog is called
   <a rel="homepage" href="http://example.org/blog/">Understanding
Semantics</a>.
</div>

Is there a technical reason why @vocab was only allowed to be a URI?

Steph.

[1] http://www.w3.org/profile/rdfa-1.1