Hi Rosea,
Not a good thing that such a core application is not in Debian stable / testing.
True, but Lenny users can get a backport of Ardour 2.7 here: http://apt.64studio.com/backports/pool/main/a/ardour/
Why does other distro's like Ubuntu, Fedora, OpenSuse have it included?
Ubuntu probably took the package from Sid, unfortunately it entered Sid after the Lenny freeze. Fedora and OpenSuse may not have the same strictness about embedded libraries that Debian has.
Cheers! Daniel