You'll find a lot of those really low hour machines are also priced a little TOO well, scams abound on this shit.
the bigger reason, NEW tractors in 2013-14 have Tier 4 emissions on them, particulate filters, and catalyst mufflers, and a lot of impatient operators.
the new stuff you can't baby like you could the older diesels, they run at higher operating temps and higher speeds or the catalyst and DPF will not function properly.
the smaller tractors don't have DEF ( under 50hp not required if I recall ) but they do still have DPF burn off events. the repairs that come with not operating properly can be aggravating and expensive.
Now, as far as OLDER machines having low hours, well, mine is 18yrs old has ~1345 hours on it. I bought it spring of 2010 with 1170hrs. I use it primarily for mowing 1.5hrs per event, but have fits where I use the loader and box blade, I let Shawn use it about 2.5-3hrs in Late April it had 1325hrs on it when he was done ( I serviced it very soon after he brought it back ) and then used the hell out of it for the next few weekends, mowing mostly ( mine and another yard, about 2acres each go)
Some folks just don't use their machines, for what ever reason
ALSO, on older machines the hour meter is part of the tachometer, you're only really getting accurate hour for hour run time at full operating RPM, baby the machine ( which isn't good for it anyway ) and it's like you're slowing the time down.
Im on Orange TractorTalks forum ( the Pirate of Kubota but a lot nicer ) A lot of folks whine about putting hours on the machine, WTF did you by it for ? yard art ?