MTB's have a place, but I'd never use one in Denver. There's just too much nice weather and too little snow.
Back to the original poster's question, why so many mtb's?
1. they are cheaper than road bikes?
2. they are cheaper than tri bikes
3. they are not as choice of a thief target as a road bike
4. it's hard to find touring bikes, and they are pricey anyway.
5. they're cheap, cheap, cheap.
Most commuters on the forum are still in school and are pinching pennies. If you're riding at school, there are no safe places to leave an expensive road bike. FYI- road bikes start at about $1,500 and can easily go to $10,000.
You want a good road bike for commuting, then consider this:
http://www.trekbikes.com/us/en/bikes...land/portland/
You'ld have to replace the idiot fenders with real fenders, add lights, different pedals, etc. You're starting at $1700 and then adding 2 to 600. Most commuters on these forums don't ride in place of paying for a gym membership, but ride to save money. They can't justify spending $2,000+ for a commute bike. Althought, they would never, never been seen driving in a $2,000 junker. Go figure.