This is what one of Eureka’s employees answered at the blu-ray.com forumshttp://forum.blu-ray.com/united-kingdom/106707-eureka-masters-cinema-moc-discussion-thread-111.html#post4767881:
The English audio was taken from a sub standard source and, as with the hd master, just wasn’t up to scratch. There was a loud audible hiss and loads of pops that try as we did we just couldn’t make it work. This release has been a real effort for all involved, we are releasing it nearly 2 years after licensing it and after ploughing thousands of pounds into it. What we’ve arrived at is a release we are happy with, we would have loved to have had a blu-ray edition with both audio tracks but at this point in time it really isn’t possible. We’ve had people all the way up the line working on it including Grimaldi’s studio and we’ve just not been able to get all the elements. Hopefully we’re releasing a disc that people will enjoy.
As I probably mentioned somewhere months ago, their standards are high so even if we here would have been happy with something that is a bit better than the Japanese disc track (or even that just cleaned up a bit) it just isnt good enough for them.