Elta
我不会把这种
"Come on Jessica, come on Tory,
let's go to the mall, you won't be so-ry"![]()

I'm reasonably certain that there is a larger population of natively English-speaking Americans than there is of natively English-speaking Britishers, you know, on the order of 'hundreds of millions more', yet is it not generally agreed that there are many more varieties of British English than there are of American English?
I might be going out on a limb here, but I think it is because British people are much more class conscious than Americans are. So the in group out group dynamic is stronger.
Related: In the past young girls in the lower classes (In England) were taught to speak "up" - That is to sound like an upper class person in order to have a better chance at marrying someone of a higher social class. There has been a notable and steady decline in this since the end of WW2.