Because it only marginally increases security as stored hash = hash2(hash1(pw, salt1), salt2) = hash'(pw, salt1, salt2). Javascript hashing can only add minimal

Because it only marginally increases security as stored hash = hash2(hash1(pw, salt1), salt2) = hash'(pw, salt1, salt2). Javascript hashing can only add minimal equivalent expensiveness of the combined hash. OTOH, once scrypt is builtin to most browser, we can offload scrypt (with large memory factor) in the browser and use a cheaper bcrypt on the server side. Again this doesn't really increase security that much besides saving server resource. One added benefit of the above approach is that it makes DDoS harder and more defensible.