My tires accelerate their rate of flatting as they age, so if your mtn bike tires are much older than your skinny ones, I would attribute it to that (as well as quality differences if any). My 2" big apples have only flatted twice in ~4k miles, from a drywall screw that drove in up to the head, and then a couple days later from a big piece of glass that found the hole left by the drywall screw. Yesterday and today I accidentally rode over the same broken beer bottle and no problem, though. Whew.