Part of the US use of revolvers was production tooling. Both Colt and Smith & Wesson already having tooling/production lines for both medium frame (.38) and large frame (.44-.45) revolvers. While Colt also had tooling for the .45 automatic. Both companies could churn out thousands of revolvers while increasing (or tooling up for) automatic pistol production.
The US forces also used large numbers of revolvers in WW II but the US, in both wars, tried to issue pistols in a range of need. As in front line troops got the .45 automatics. Vehicle, gun crew or supply troops got .45 revolvers IF not enough 1911s in the area. Home front factory guards, military policemen, and auxiliary troops got .38 special revolvers. A lot of aircrew got .38 revolvers (they were smaller/lighter than the .45 revolvers)
The US Armies use of the 1911 .45 automatic was not particularly eye opening as they had been working towards it for quite a number of years.
Colt 1905
developed from the 1902 model in .38 automatic.
The US Army had pulled the .38 long Colt revolvers from service and re-issued the old .45 Colt single action revolvers after the experience in the Philippines. A number of Companies submitted prototype .45 automatics in Army trials before the M1911 was adopted so it didn't exactly take the world by surprise.
Not to mention that most of Europe was buying automatics as fast as they could from about 1906 on, except for the French and British.
And that is
service automatics and not pocket pistols which only entered military service in 1914/15 due to shortages.
as far as this statement goes:
"I would concede the 1911 as perhaps the best side arm of ww1 but I would be not keen on saying it's the best of the C20th."
I don't believe there is any
perhaps about it unless you can come up with a better pistol in WW I?
The 1911 (or 1911A1) is also a contender for best pistol of WW II with only two viable challengers, The German P-38 and Browning Hi-power.
I would also note that any other viable challengers only appeared in the 1980s.