Why do only US makers focus on time coherency?

Most of the research I've seen (admittedly not much), suggests that phase coherence is not audibly significant except in extreme cases. That's not to say this is a settled issue, but it would explain the tendency of many companies to deploy their R&D dollars elsewhere.

If you were to close your eyes at a live concert, you'd be surprised at how hard it is to pinpoint the position of instruments/voices. With your eyes open, however, it's quite easy. (A lot of things are easier with your eyes open.) Good imaging at home helps to make up for the lack of visual cues.

Of course, good imaging is a function of a number of factors, including the recording and speaker-room interaction, as well as the listener's own sense of what it _should_ sound like. I'd be wary of any dogmatic claims that "X" type of speaker is the best for imaging.

Why don't you just forget about speaker design, listen to a bunch of stuff, and decide what you like? There is no best speaker design, just the best speaker for you and your room.