This is linked into the motivations for v1 of the API.
First, we wanted to put everything you could scrape into the API, so there'd be less motivation to do so.
Second, we wanted it to be possible to build read-only versions of the sites using just the API. The theory being that if this were possible, the majority of interesting [app]s would be possible. This is why the (now quite out-classed by the community) WWSOC was our example app.
Naturally, this is not an exhaustive list (leaving out things like "scalable" and "flexible").
/revisions, and probably a few more I'm forgetting) don't really jive with the above. You can't get a stream of new comments or answers anywhere.
Now, that's not to say there never will be an
/answers just that it wasn't considered for inclusion into v1. In general, new ways to slice and dice data are great candidates for subsequent API versions.