We have to keep in mind that there were virtually no surgeons in the U.S. performing FUE until roughly 2004. Then, as some of you may remember, FUE hit North America like a tidal wave!
The 1 mm punch is the one that most surgeons favored to learn and refine their technique and skill, for the obvious reasons. So there was quite a lot of havoc and at times panic in some of the initial results, both in terms of yield and donor scarring.
Over the years, and as the degree of proficiency and skill advanced, many of them started to use smaller sizes as long as the rate of transection was not compromised. Many patients have lots of single hair FUs and doubles...theses are not as difficult to extract as compared to multiple hair bearing FUs, all other factors considered equal (proximity of FUs to each other, scalp texture, etc). So the larger FUs may require a larger punch size for safe extraction.
Unfortunately there are some who continue to use the 1mm and even larger sizes because the manual extraction process goes faster, and time is money.