Until Hideyoshi's laws about swords and wearing of swords c. 1590, there would be a lot of variety. After Hideyoshi's laws, daisho (daito and shoto, worn together) were restricted to samurai (and early in the Edo period, became required wear for samurai on duty), and there were restrictions on weapons ownership by civilians. From 1668, commoners could not wear swords longer than 1.5 shaku (ko-wakzisashi, or short wakizashi), unless given specific permission by a government official.

Before Hideyoshi's laws, civilians could and did wear daito, so no surprise to see ashigaru with daito. In comtemporary art, I've seen ashigaru with katana and tanto, with 2 katanas, with daisho, with tanto, with shoto. One interesting picture of ashigaru musketeers training shows them with swords of about 2 shaku (so either very short daito or very long shoto), with the officers supervising them wearing longer daito.

They would usually have had at least one sidearm. Their main weapon would have been yari (spear or pike) or musket.

I'd say as others did convenience. A single handed sword can form part of regular dress or ''regular carry'', as modern police say about handguns , which means they will be a lot more popular than long arms. Keep in mind any weapon, including modern soldiering you generally carry it 99.99% of the time versus less than 1% of the time actually needing it. As such even the toughest knight in the toughest time period would not be wearing his two handed sword to the toilet or dinner.

Also in my own experiements, taking swords out to test cut wild pigs or goats on we have just shot in Australia and the novelty of lugging a hand an a halfer through the bush gets annoying about oh,,half way through the first day! especially having to take it off your back to sit down and lean back to rest. A type X or messer hangs nicely and you forget its there.

I just wrote this piece about the longsword on my blog. I remain uncertain exactly how the longsword and similar long-gripped swords compare with single-handed swords. I remain uncertain exactly why longswords and company saw use historically and why they didn't.