Why do only US makers focus on time coherency?

That's a very good question. The british speakers seem to be more interested in smooth frequency response, than time and phase. To date I haven't heard a british speaker that images particularly well (and don't say ls3/5 because they don't image at all !)

As a brit myself I find my compatriots' fascination with this thing called PRAT to be a bit odd. Why are we so quick to throw out spatial information to try to make the music bouncy ? Perhaps because fewer brits have the space in their houses to allow for speaker positioning that would allow imaging ?

The new Triangle tweeters were developed as a semi horn design to address this issue in a more conventional cabinet.

My Coincident Digital Masters are time coherent, but Israel has gone to more conventional cabinet designs in his recent efforts to ease manufacture and lower costs.

Reference 3A speakers claim time coherence.

My friend Michael's (Sufentanil) Audio Physics speakers are time coherent.

The Fried Valhallas (C Series monitors) were time coherent, and we are probably going to introduce a C7 monitor next year. The pair we are using as our reference to the old days can really play tricks on your ears, they're flat out wicked! I once SWORE a phone was ringing behind me(and I was sitting against a wall), and it was the music - speakers were 15 feet in front of me.

But, overall, I think your premise seems to hold water. It's an interesting point, thanks for raising it.

Sean- what is prat?
hammy

Hammy .. Pace Rhythm And Timing. It's about whether a stereo makes you want to tap your foot or go to sleep.

Bombaywalla is incorrect-JmLabs speakers are time AND phase correct. They design the structure of their speakers and their crossovers with that in mind. Don't forget that it all depends on listing distance and height of the listener relative to the loudspeaker drivers(especially the tweeter) in order to benefit from such a design.
Wilson Audio's are also correct, and are set up when you purchase them for your arrangement, but then again, they're American also.