Why not? I think there are a lot areas in which MUni is easier than MTB, not sure we can get to 101 but here it goes:
Hike-A-Uni is a lot easier than Hike-A-Bike.
Pre-ride prep/check (if you do this) is typically much quicker.
Post-ride cleanup/check (if you do this) is typically much quicker.
Transporting the Uni is much a snap, just throw it in the trunk or backseat, no special bike rack required.
Fewer moving parts means lower possibility of mechanical failures.
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I was thinking this topic needed to be said:
Its QUIET
Hike-A-Uni is a lot easier than Hike-A-Bike.
Pre-ride prep/check (if you do this) is typically much quicker.
Post-ride cleanup/check (if you do this) is typically much quicker.
Transporting the Uni is much a snap, just throw it in the trunk or backseat, no special bike rack required.
Fewer moving parts means lower possibility of mechanical failures.
Easier to fall and land on your feet
Every comment (except “wyow” perhaps) tends to be encouraging
Skinnies are easier - only one tire to worry about
Tight turns are easier.
Hopping over stuff is easier (well, in theory, anyhow)
You don’t go as fast.
Less dangerous, in general.
Even short local trails can be fun/challenging, so you don’t need to drive as far for a good ride.
You always get better, every ride, and that’s SOOO encouraging. (perhaps this is the case for MTB as well?)
its super quiet
you only have to hit every bump/drop once, rather than twice.
You get a workout downhill as well as uphill.
Can sneak a smaller wheel on an airplane to avoid extra big cost.
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You only have to worry about one wheel.
After a muddy ride you have to clean less parts.
In the desert you only have to fix one flat instead of two.
Your chain is never going to break.
Same for derailleurs and assorted other parts.
It’s a lot easier for a uni to fall off a cliff without you on it.
I’ll add my top reasons:
fewer parts, more reliable
lighter
easier to carry (because of 2, and because of shape)
brakes (if you use them), have a backup system in the fixed wheel
People must have a different attitude where you are! I’d say one of the bad things about unicycling is the constant mickey-taking (although some people are impressed, to be fair).
I think muni is certainly SAFER than mountain biking - you’re not going so fast and it’s way easier to bail cleanly if you get it wrong. I’ll ride stuff on my muni that I’d think twice about on a bike. Not sure about EASIER though, unless you count lower maintenance and ease of carrying over stuff you can’t ride. I definitely reckon it uses more energy than it would on a bike to ride the same terrain (and slower).
Rob
I used to think the same thing, but after sustaining a stress fracture to my heel while riding MUni on the Slickrock trail in Moab, Utah, I feel different about that. I think overall the risk of injury on a unicycle is less than on a mountain bike, it isn’t zero. Certainly the types of injuries sustained by mountain bikers aren’t as likely for mountain unicyclists, however there are still many ways to injure yourself riding off road on a unicycle.
My advice is to make sure you’re wearing the proper safety gear (armor, helmet, shoes) when riding off road. In my case I wasn’t wearing my good off road shoes (left them at home and bought a cheap pair of shoes) and I suffered the consequences! I might have broken my foot even with the proper shoes, but I think my risk would have been far less.
Be careful out there!
Like a friend of mine recently said:“You get introduced to a whole new set of injuries when mountain unicycling.”
Still worth it though. 
OK, I said safer, not utterly free of risk 
Most of my local trails are very rocky but nothing “extreme”. They get pretty easy on a bike when you’re used to them - so the tendency is to ride faster and faster because you can on a bike - then a crash is really quite nasty. On a muni it’s more of a challenge anyway, and you’re going more slowly - you’re much more likely to fall off than a biker, but when you do it’s much less nasty. Of course, you could fall awkwardly or sprain your ankle as you put your foot down but you could do that while walking or running. Having said that, I wear shin/knee pads, gloves, helmet and elbow pads when unicycling on the rocks - on a bike I never wore any protective kit other than helmet and gloves. FREQUENCY of falls from a unicycle are higher than from a bike, but SEVERITY is much lower - at least IME.
Rob
Yep, it was exactly that that did me in! For over 6 years, probably 80% or more of the time when I come off the first point of impact was onto my right foot, typically right on my right heel. Do that 100s if not 1000s of times, again and again, and eventually you weaken… So on a fairly typical UPD in Moab, nothing spectacular, just off the uni and onto that right foot and WHAM stress fracture! Which is why I say we unicyclists have to be aware of repetitive use and abuse injuries such as this. So just be cautious out there (or learn how to bail off onto your other foot to balance out the abuse! 
).
I found that if I increased my core strenth (especially leg lifts) it increased the likely hood of landing on my feet. If I worked my less dominant UPD leg so it was stronger, it was easier to land on that foot, and if on semi intentional falls I tried to land on that leg, when the UPD was totally unexpected I was more likely to land on that other foot.
I’m pretty sure Brent was referring to riding on trails, or in other MUni-type situations. That’s where this is true for me also. The farther you get from the “casual observer”, the better the comments. Ride a little trail in Central Park, for instance, and you’re fair game for all of it. But ride a remote trail above Lake Tahoe, for example, and the odds of getting annoying comments, on the trail, are near zero in my experience.
That’s exactly the problem with my left knee. I wear good shoes, so the next weakest point gets injured. Must still learn to UPD on the right leg.
Hmmm… lately it’s been much, much better but there for a while I hit nothing but the “all will bow before me” cyclists that wouldn’t even make an attempt to meet me halfway. I’d get on the edge of the trail, they would make no concession whatsoever, then I ended up having to go off-off-road.
Happened quite a bit at one of our local single track spots. I HATE going there after the crowds show up.
Like I said, it’s been a much nicer crowd of late, maybe only the die-hards are out in the colder weather and maybe the die-hards are more courteous.