Super interesting question that I have tried to investigate myself on several occasions.
I found an amazing article released on LegalBusiness which discussed this (
https://www.legalbusiness.co.uk/blogs/guest-post-the-magic-circle-is-doomed-heres-why/).
Taking a look at all the MCs except for Slaughter & May, there are a variety of factors as to why the firms are struggling to break into New York's legal market.
Clifford Chance (CC) tried in 1999, merging with PE speciality firm Roger & Wells, resulting in huge management issues and breakdowns in communication between the two main offices in London and New York (
https://www.independent.co.uk/news/uk/crime/lawyers-divided-by-a-common-language-7907434.html).
Below are a few reasons as to why UK firms struggle as aptly put in that first article link:
(1) Profitability
NY-based firms or the traditional 'White Shoe' firms are more profitable on average. This article quantifies this well
https://adamsmithesq.com/2014/08/the-elite-face-off-ny-vs-london/4/ (keep in mind it's from 2014 so a bit outdated but still relevant).
(2) Salary - which is tied to profitability
I think this summer is a good example as to one of the main reasons why UK firms in general will struggle in the US market. With the recent massive NQ/TC salary increases from Milbank, Akin Gump,
Latham & Watkins,
Vinson & Elkins and
Dechert (there are more than just these firms) this summer in London, we can see that US firms are willing to generously remunerate when compared to its UK counterparts.
Also, many star partners at the New York elite have bespoke remuneration packages with the firms. UK firms would struggle to poach partners from these firms and match the salaries/bonus structures being offered.
(3) US Litigation
This is a new issue that has cropped up this past decade. The US Tort industry is growing steadily every year, pushing in excess of $300bn yearly. This litigation alone, not mentioning highly niche and lucrative specialities such as patent litigation and labor litigation, account for 30% of major US firm income, a number which is half that for UK firms.
Despite the dominance of the English bar, UK firms entering the US would need to bolster expertise in these litigious practices to compete. This is tied to the two prior points of salary (as UK firms would need to recruit US associates/partners to develop these practice areas, which is also dependent on profitability).
And then vice-versa, the two points reign true as to why US firms conversely have been successful in the London Legal market. Interested to see what others have to say about what else separates these two types of firms in the different legal markets.