There are a couple things to note here. Firstly, 1983 was the first ever World Champs for Track and Field that following August in Helsinki.
The Boston marathon was the US Trials/Championships to determine who would make the team.
And because the US had boycotted the 1980 Olympics, this was the first international championship in 7 years that the US was going to compete in.
So combine those two factors plus it was the BOSTON marathon and EVERYONE geared for it that year!!!! And with the US hosting the Olympics in 1984 everyone was gunning for the marathon trials standard as well.
Check the results of the years around 1983 and it looks more like 2006 except no 2:07.
My old college coach Fred Torneden actually ran in this 1983 race and yes he is the top 50. He said he was a bit off that day in Boston, had been running around 2:14-15ish in other marathons. He ran a PB the following year in 1984 to win Twin Cities in 2:11:35, the same exact time Dan Browne ran there to win in 2002.
If every year was "2006" then a lot more US guys would run Boston each year, but because now every year except 06, 10, 14 etc. has a World Champs or Olympics that means you have to run in the US Champs or Olympic Trials wherever they are to be in consideration for a World Champs or Olympics team. That doesn't leave a lot of room for other marathons to be run in one year.
Just the facts, if Boston was the US Champs or Olympic Trials every year then there would be a lot more US guys there, but that's not the way it is.
And as for courses, weather, etc. you CAN'T compare them because each day is different. How about 1982 Salazar vs. Beardsly, pretty warm day. Great performances that would've been even better without the heat. Or on the other end how about 1994 when there was a massive tailwind and Cosmos set the record in 2:07:15 or what about 1986 when DeCastella ran 2:08. How can you compare them? You can't.
Fact is that in 2006 there were 5 US guys in the top 10 and they all ran tough and smart races to finish where they did. Nobody gave them that. They ran great and made the most of the race on that day. That's all the results tell us.