Why do only US makers focus on time coherency?

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.

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.