for me, it was a case of simplicity and reliability, plus if you get your gearing correct, you really dont need as many gears as you think.
i come from the bmx world, where gears arent an option. you simply learn to get the best out of your system as it is. if you want to go faster you either learn to spin quicker, or go up a tooth size on the chainring, or down one at the freewheel if you can.
it took me a while to get the gearing right on my bike, its a 24" wheeled mountain bike and i run 17/39 with 175mm cranks. short enough to have good accelleration and be able to crank it up hills stood up, but long enough to get a good turn of speed when i spin it up.
for me the issues is, no matter how good manufacturers try to make them, rear mechs are still pretty fragile items mounted in quite a stupid/vunerable position. theyre a real hangover from when mountain bikes were little more than converted/beefed up road bikes, and one thing they really should have improved upon by now. one good knock or a stick through the chain/cage setup is all it takes to mangle the whole lot to the point its unridable, and leave you with a long walk home.
add to that the use of bowden cables to control the shifting, which really do not mix well with mud and water, and its a wonder they work as well as they do.
i ride freeride, jumps, street, bmx tracks, and northshore kind of stuff, and i find having a rear mech and chain flapping around all over the place whist doing it quite annoying- particularly as your feet are stationary most of the time if youre hitting obstacles so theres no need to pedal anyway, so it doesnt really matter if youve outrun your gearing or not. then theres the added expense of a decent chaindevice to keep your chain on at the front too. the 'epiphany' for me was when i wore the same sprocket out first on my last two cassettes, as i was riding soley in that gear at least 90% of the time. to me it was obvious the added weight and complexity of gearing was pointless if i wasnt using it!!
singlespeed is just so much stronger, cheaper, quieter, and less damaging to other components (chain slap). im not sure how id get on with it for XC use, always being in the saddle, but for what i do, its ideal.
as a footnote, i see the whole singlespeed/ridgid thing going on at the minute (well, its been building for years really) as a bit of a backlash against the big manufactures constantly trying to shove their 'new' technology down our throats, basically then telling us the stuff weve already bought off them last year is now old hat.
the truth is, having one more gear, 10mm more travel, or slightly sharper disc brakes doesnt make you a better rider- its experience and skills that are built on the trail that do,and no amount of money thrown at equipment will change that. mtbs have come so far in their evolution that you can easily go out and have just as good a time on a £500 bike as you can on a £5000 bike without it breaking like they used to, so theyve got to try to sell you their wares some other way.
this is why.