it's because my roadbike would be bliss if it weren't for:

-having to unclip and put my foot down to come to a stop.
-having to adjust, maintain, own, and operate my derailleurs. the cables may be rubbing on the housing but whatever the cause, sometimes they just simply don't shift when i ask them. sometimes it's a fraction of a second, sometimes ten or more seconds before they decide to work again.
-slowing down to make hills easier. the temptation of the luxury of slowing down and spinning a lower gear is nearly inescapable. when you do give in, spinning the lower gear actually makes the hill more tedious and painful because when you give up you drop your cadence to the point where the gear is too high anyway. unless you have a megarange gear that pushes at like 24 front 32 rear (my girlfriend's cruiser raleigh c40 has this gear, it's almost comical to use, but sheer bliss when you want to move the bike uphill with a full load and all day to spin like a madman), even the lowest of gears becomes too high if you have 'given up' and dropped your momentum below the critical amount for whatever hill you're climbing.

i don't mind having lots of speeds... but the complex system of changing gears is expensive, difficult, and requires frequent service and replacement.

having one speed makes you deal with reality. whether it's fixed or not, one speed says "you either keep the bike moving or it makes more sense to get off and walk." which is really what it comes down to after all.

either your legs are strong enough to get you over the terrain you want to cross, or they're not. if you don't have to climb hills, one gear ratio should be all you need... you can vary your cadence enough to travel at a great enough difference in speeds. and if you can coast, then going down hills isn't a problem either, provided your brakes can stop you at the bottom.

if you want a bike that will go up and down hills, you either have multiple gears and good brakes or you are an adrenaline junkie with a deathwish. and let's face it, if your calves are big enough, that's a pretty cool thing to be.

if you want a bike that looks good, is mechanically simple, reliable, and cheap, and fun to ride around town where it's generally flat, you are either trying to go too fast for city/town roads, or choosing a bike that only allows you what you need, and does away with all the crap that will only get in your way.

i love my geared road bike... but if i lived where there were no hills and rides lasted less than 20 miles, i don't think i'd even keep it.