It may be that Google wrote the app using the NDK rather than the SDK. The NDK gives developers the ability to target ARM v7 processors and above; the Eris is ARM v6.
See What is the NDK? | Android Developers
That said, it may also be that Google "knows" that the Eris is limited to Android 2.1, so will not make the app available in the market.
I think that there are .apks out and about; you can try installing one of those.
The Amazon MP3 app seems to work very nicely, by the way.