Why do only US makers focus on time coherency?

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.

Shayne, didn't JAs analysis in a Stereophile write up show the JMlabs were actually NOT time coherent, despite being advertised as such?

I know Richard Vandersteen, as well as Bud Fried, are very adamant as to the importance of time coherence.

The JM Labs are not time and phase coherent by any stretch of the imagination! Look at the step response of the Utopia, the woofer is connected in reverse polarity to the mids and tweeter. The Wilsons are not time and phase correct either. Look at the step response from the Sophia.
They're very few speakers that are time and phase correct or even just time correct.
Even if you use a single driver, this in and of itself does not mean the speaker is time correct. You can look this up in previous threads.
Vandersteen, Thiel, Meadowlark, Green Mt. and few others pass the step response test indicating time alignment.
It is hard to design these speakers and the engineering costs are high. Why do it if you can sell non-time aligned speakers and discount time as inconsequential as reported in Stereophile. If you put the Vandersteen 3a Signature in a nice wood cabinet, it would sell for $10,000! Thiel does and look at their cost.
Time speaks of when a sound leaves a driver. If it is not a first order crossover design, then it is not going to be time aligned.

Why do some companies who produce highly regarded speakers...I wont name names...seem to discount time coherency...while others base their whole design on retaining it? It baffles me(no pun intended!)...

Phasecorrect, don't forget about Thiel!