Further Investigation clears up some of the mystery.

The following ML post [1] claims that Rant
covers some major use-cases which Rake doesn't. Key quote: "Rake
appears to have no concern with anything outside the Ruby community".
In other words, Rake is dedicated to building Ruby software system,
and not interested in building anything else, while Rant is.

The author of that post was familiar with Rake [2], and later became a
lead developer of Rant. So I assume he knows what he's talking about.

That said, the above provides a solid conceptual ground for the
overwhelming preference of Rake over Rant in both Rails and the static
site generation systems I'm currently reviewing, all of which are
focused on very simple and/or Ruby-specific tasks.

As for more practical grounds, this ML post [3] reveals that as recent
as six months ago, Rant was experiencing serious existential threats.
The sole developer announced he ran out of time and was not planning
any further development. As Rant was at that point (and still is) in
beta state, stagnation meant death. And indeed the major discussion
channel of the Rant community was fizzling out.

Under such conditions, it is no wonder that Rant would be perceived as
too risky a choice for anyone but those who actually needed the
non-Ruby-related features it offered over the stable, mature and vital
Rake. At least three of these later sort joined the Rant development
team soon after the quoted discussion took place. Since then, Rant has
been making pretty steady progress. I'm not sure it would become a
tempting alternative for those who can do with Rake, but it certainly
looks poised to becoming more appealing for those who need the extra
features and/or are looking for a build tool for non-Ruby-related
systems.

-Chris

[1] http://rubyforge.org/pipermail/make-cafe/2006-August/000089.html

Most of the information relevant to our discussion is summarised in
the the second-to-last paragraph:

"So I have arrived at Rant.  Based on Ruby so easy to work with.  Almost
but not quite a superset of Rake.  Interested in building things other
than Ruby software systems.  Seems like a better choice than Rake.  But
is it?  This is a question I have to ask myself and have a realistic
answer; I need to feel comfortable about investing the effort to move
from Rake to Rant.  Apart from the fact that Rake has an article by
Martin Fowler and is deeply internalized with the Ruby community, it is
stable.  It doesn't seem to have any community but it does do the job
and lots of people are downloading it.  Rant appears to be technically
superior but it has no community and no publicity machine.  The analogy
here is between VHS and Betamax -- VHS was inferior but won out because
of the publicity and expectation management.  Currently Rake is winning
over Rant because it appears as a download on the Ruby front page."

[2] http://rubyforge.org/pipermail/make-cafe/2006-August/000083.html
[3] http://rubyforge.org/pipermail/make-cafe/2006-August/000086.html