that you don't speak the L in solder is completely new to me, what would be the reason?
Well, the obvious answer is several hundred years of separation before we had any form of telephony or occasion to hear it spoken.
Take Newfoundland (a province of Canada), for example. There are Irish accents which haven't existed in Ireland for a few hundred years. Why? Because they were remote places, without a lot of interaction, and the accent remained intact even after it had died out in County Cork.
Why is there High German and Low German? Again, either geography, class, or something else. My guess is mostly class.
In North America you're talking about a huge land area, settled over time by people from all sorts of places, and with no fast transportation between them.
Even regionally the pronunciation can change quite a bit. Heck, I don't have the same inflections as most of my family does. I've actually been asked where I'm from by people who grew up in the same area.
In the UK, there's massive differences in accents -- some class based, apparently, and some region based. There are plenty of people in the UK who, as far as I understand, will say "hise" instead of "house". Why that would be, I have no idea.
For instance, I have known more than a few Brits who almost can't say "th". So, instead of "thought", it comes out as "fought". They don't even hold their mouth/tongue the way you would to make the "th" sound, they make an entirely different sound, essentially the way I'd make an "f" sound. Similarly, in my limited experience, a lot of German speakers turn the "th" into more of a "z" sound, so you can get "zinking" instead of "thinking".
Beyond that, you'd need to ask a linguist. Over time, what you grew up hearing and saying defines how you say it, and what you can say. I strongly suspect there are some sounds that some people simply cannot make if they didn't learn early enough.
Sometimes, I do wonder if accents aren't sometimes the equivalent of a regional speech impediment, and they will always affect how you speak. :-P
The longer I live, and the more non-native speakers of English I meet, the more I have a hard time explaining all of the corner cases in English, because it's made up of stuff from so many different languages.