I am not an authority on the reasoning behind it, perhaps a person with vision to the decisions made along the way can step in, but from what I can tell, var was originally a location for any temporary or derived data, but not necessarily customer facing.
Versioning, and a stack of other information is stored there, so it should not be exposed to clients.
/etc is the location that has been left open on the dispatcher generally, so the logical place to put lookup info if it belongs as part of the SDLC, is within this location.
If the lookups were to be authorable, it would be more wise to place it in the /content directory, which if you're doing anything that involves multitenancy will fit in with the rewrite strategy. Furthermore, if different tenants are accessing the same lookup data, ie,
http://www.mysite.com/mypage.html goes to /content/mysite/mypage and
http://www.anothersite.com/anotherpage.html goes to /content/anothersite/anotherpage,
a rewrite rule can be used for the shared location. e.g.
www.mysite.com/lookups/postcodes.json /content/lookups/postcodes
www.anothersite.com/lookups/postcodes.json /content/lookups/postcodes
Hope that makes sense.