In reply to Andy Long: Superb question. I don't know either.
My best guess is that after ovals screwgates were eclipsed by stronger D then offset D krabs it meant that at some point there was only really a choice between high quality HMSs or Offset Ds. Between these two, the HMS is better, especially for 2 ropes, hence it possibly became the default...
Interested to hear any other suggestions.
I'm currently using a DMM Sentinel Locksafe and find that it is substantially better than the previous large HMS krabs I used for many years. Why I ever thought a DMM Boa was a good belay biner, I don't know...
I completely agree that there is a compelling case that modern oval screwgates make an excellent belay biner. If I have the chance to get hold of (or borrow) one, I intend on having a play and see whether it improves on the Sentinel.
In that vein of experimentation I recently had a play with using a DMM Belaymaster out trad multi-pitch and found it relatively workable - still a minor faff, but not one that I couldn't live with. Given that you can rarely afford the luxury of multiple sets of equipment, I'd be happy to issue them to students going trad climbing as well as just using them for novice groups where I feel they do make belaying marginally easier, both for students and the instructor.
I'm also intrigued by the Black Diamond Gridlock. Again I've like to see if it can cope with trad multi-pitch as well as being good for indoor walls use with novices. My gut feeling is that it would fair even worse than the Belaymaster, but I would be happy to be proved wrong.