I'm aware that US English is supposed to be similar to 17th century British and that it's changed less, i.e. more true to the original form, but there are some things which don't add up. Why would American English remain the same for 400 years?
Nobody said it remained the same. I expressly said it changed
less, not that it didn’t change at all.
Why did British English change more than American English? Could it be that British English had more influence from Europe?
That I don’t know. But as already discussed above by several people, it seems to be a common pattern in colonies of varying languages.
How about T dropping? It has been common in American English for a long time, yet it has only become widespread in British English fairly recently. 100 years ago it was restricted to a few small parts of country i.e north-east England, East Anglia and east London, but now it's everywhere. It's also different to the US: Brits will tend to use the glottal stop, so computer becomes compu'er, whist the Americans will replace the t with soft d: compu'er, although the Brits occasionally use the soft d too. I find it hard to believe that T dropping was widespread in England in the 17th century, it dying out in and re-emerging recently. I suspect T dropping was not common in 17th century English and developed later in America.
Well it’s not “dropped”, it’s morphed into other sounds, namely the
glottal stop as you mentioned, and the
alveolar flap (what you’re calling a soft D).
I am not as well versed on the history of this specific change in English (and my reference book doesn’t go into it), but chances are that this modification appeared in some regional dialect of British English centuries ago, and only comparatively recently
spread. 400 years ago, the regional dialects in Britain were probably even more pronounced than they are now, and we know that the differences in the regional variants of American English are strongly informed by
when the particular English (or later, Irish and Scottish) settlers came, and
where they came from specifically, since emigration happened from different areas in England at very different times and with different destinations.
We’d have to know what the various regional dialects in Britain were like 400 years ago, and how they morphed over time.