That's a great question!
Just speaking about the fifties...
I'd love to think it was because of a sporting heritage or something like that but my short answer would be that the preponderance of 2 seaters was due mostly to immediate postwar government policies that forced companies to "export or die". In 1945, steel was channeled to companies exporting over half their production and this was raised to 75% in 1947. By 1950 about 75% of all cars made in England were exported.
The US market was booming, MG showed that 2 seaters sold well here and Austin, Standard, et al followed suit. With exceptions, most 2 seaters made in the 50's were sent to the US and other markets. Most British cars were not well suited to our driving conditions and dealer support was pitiful. Sporty car buyers accepted these problems because the cars were fun and overlooked the poor parts availability and reliability problems.
So, 2 seaters were built because they were most suitable for export.
Also, once you get past the high volume 2 seaters, production numbers get pretty low. I'm not sure 2 seaters were made in much greater proportions than in the US. We had our low volume 2 seaters also.
Anyway - that's my short answer. In the middle of all this, too, were currency devaluations, other government initiatives, and more exciting stuff.
I think the 2 seaters built in Britain from roughly '65 on were pretty much products of inertia. Few major development efforts were made in this area after that.
I'd be interested in the perspective of the some of the Brits on the forum.