arm ldxr/stxr vs cas

I read that arm added the cas instruction because they didn't think
ldxr/stxr would scale well.  It wasn't clear to me as to why that
would be the case.  I would think the memory lock mechanism would
have really low overhead vs cas having to do an interlocked load
and store.  Unless maybe the memory lock size might be large
enough to cause false sharing issues.  Any ideas?
Joe Seigh