The problem is that what is holding Scrypt back is not what was holding SHA-256 back. You can throw more and more processors at SHA-256 and come out with more and more hashrate. This is simply not true for Scrypt. Scrypt requires more processors combined with a higher and higher memory bandwidth. Nvidia and AMD place a lot of money into memory R&D to that end, more than any person interested in a Scrypt ASIC can currently field.
Basically, a Scrypt ASIC will not hash faster than a GPU, because you're not going to put >GDRR5 memory on an ASIC. However, a Scrypt ASIC could work as a standalone unit, which would be beneficial for someone who wants a lot of them. Other than that, there is no real benefit to designing a Scrypt ASIC for anyone interested in cryptocurrency.