Oh I am going to catch hell for what my theory is and it is not just musicals, but all theatre.
UK - Intellect; the shows come from the head
US - Emotion; the shows come from the heart.
Musicals almost by definition have to come more from the heart than the head ("Its the MUSIC, stupid."). But if your theatrical thinking puts theatre into your head, you strip an essential away from emotion-centered shows. And vice versa.
The easiest example is to compare how Neil Simon has rarely found success in Britian and Alan Ayckbourn has rarely had it in the US.
I am not a fan of the stagings of Sweeney or Company. I wanted the dread, the shock, the blood and the bodies piling up in Sweeney. To take the horror and gore out of Sweeney oddly takes away it's emotional core.
Company, a show with limited heart anyway, had all of it removed. Robert is not involved in a show that revolves around him. Where is his humor, charm and intelligence? His wit? His irony? Instead we are given a depressive alcoholic who has no fascination. Gower Champion's rule was "Who do you root for?" I really didn't care much for this Robert at all. Can you imagine being trapped next to him at dinner?
At Company's core is Robert's loneliness and up to Side by Side by Side we see him dealing with being alone quite nicely. But then some shade begins to darken the landscape. This Robert is far too moody all night long.
Certainly there are intellectual shows that have played well in the US. Plays by Peter Shaffer and Tom Stoppard come to mind. Arsenic and Old Lace was a London smash.
Me and My Girl got it pretty good. Les Miserables too except that before it 'became' British, Les Mis was French.
Passion will always play better in London as will Sunday in the Park as they are both cerebral works.
Leave as much feeling in a musical as you can. Sweeney failed for me and Company kind of does too.
"If my life weren't funny, it would just be true. And that would be unacceptable."
--Carrie Fisher