Bikes with two or three chainrings are the norm, and then there are singlespeeds, but except for kids' bikes, you see almost no stock bikes with a single chainring in front but a cassette and derailleur in the back. It seems like for a lot of people this would be a nice balance with respect to simplicity, weight, maintenance, so why are there so few?
1x9 setups are more common on a mountain bike or commuter bike setup with rapidfire shifters than a drop bar setup. However, due to a lack of front derailleur they can have chain jump issues due to the effect of the rear derailleur on the chainline. This has to be compensated for, often with a chain guard on the outer side and jump stop on the inner side of the front chainring. It's also a bit weirder to have 2 STI brifters and only have one of them for shifting on a road / cross bike. Especially when brifters cost a lot of money.
Most groupsets come with all the drivetrain and shifting gear for a 2x9 or 2x10 setup. So, they install it on the bikes as a set, It would probably be cost prohibitive for manufacturers to break up the groupset and they would probably have to pay for extra parts (like a chain guard) to make it work.
There are however several bikes on the market with a single front chainring and an internally geared rear hub. These bikes don't suffer the downfalls of having to deal with chain jumps as much as bikes with derailleurs since the chainline stays consistent (and hopefully releativly straight).
UPDATE: Modern mountain bikes and some cyclocross bikes have been switching to 1x drivetrains as a standard drivetrain option. New technology, clutch rear derailleurs, narrow wide chainrings (there are several variations), and wider range rear cassettes (up to 44t) have made it so you can have a wider gear range and get rid of the extra chain retention gear that you used to need for this setup. SRAM and Shimano both sell 1x drivetrain groupsets and now some frame manufacturers are dropping support for front derailleurs.
There are plenty of bikes available with a single chainring. However, only certain types of bicycles come with single chainrings. People generally buy according to their needs, and products are generally offered in accordance with demand.
The two main categories of bikes that often come with a single chainring are comfort/city bikes and track/single-speed bikes. Comfort bikes such as beach cruisers often have coaster brakes, chain guards, and other features to minimize the amount of maintenance required. Track bikes were originally designed for racing at velodromes, but they have developed a broader appeal due to trendiness and low maintenance requirements.
On the other hand, road bikes and mountain bikes generally have a wide range of gears to allow a wide range of speeds. If they came with only a single chainring, they would not meet the needs of most cyclists, so they wouldn't be bought.
In short, there are plenty of bicycles with single chainrings, and they tend to be bikes that are designed to be low maintenance. Bikes designed for performance usually come with a wide range of gears.
I suspect marketing. More is always better. If you were naive to bikes and you saw two derailleur bikes for $499: One with 9 gears (1 x 9) or one with 27 gears (3 x 9). Which do you think they will choose? In western society, more is always and without question better. The single chainring bike might have better components (due to being simpler at the same price point), but people will fixate on the number of gears.
That said, I have noticed quite a few 8 speed hub internal hub bikes on the market now. Single chain ring with essentially the same number of gears as a single chainring derailleur bike, but they can do a marketing end run around the issue of number of gears with a different sell, namely the reduced maintenance and simplicity associated with an internal hub.
Aside - I will avoid any discussion of whether or not internal hubs are simpler and lower maintenance, just that this is the common "knowledge" about internal hub bikes.
UPDATE well the market forces have swung and now many high end bikes are switching to 1x (11 to 13 speed cassettes make this feasible), so more 1x are now finding their way to lower end bikes.
A wide range of gears on the rear is 11 T to 34 T which lets you vary your mechanical advantage by about 310% on a bike with only a rear derailleur.
The front derailleur is (compared to the rear derailleur) a simple piece of equipment that greatly increases your ability to go fast on the flats while climbing comfortably. Adding a front crankset that goes from 24 T to 52 T lets you vary your mechanical advantage by ((52/11) / (24/34)) or 670% with both derailleurs between large-front/small-rear and small-front/large-rear.
(The actual mechanical advantage has to take into account the ratio between the pedal circle circumference and the drive-wheel circumference, but I'm dividing mechanical advantages here so that constant divides out.)
Bikes with only a front derailleur aren't made today because they don't pick up the slack in the chain, though I've heard that early model chained bikes were often ridden with slack chains.
In case of road bikes, back in 2012 (when question was asked), there was simply no option. There was some cyclocross setup but didn't really sell in this market.
Road single chainring group set only available in 2015 by SRAM. I just switched to their 1x11 (48t 11-36) from 2x10 (50/36 11-26). Hardly miss the closer ratio of the 2x10 or top speed, which probably would be important if you race. I really appreciate the easier effort needed to clean bottom bracket area.
I can see that 1x12 setup is coming, so expect to see more people switch from double chainrings. And then we'll probably can have single chainring specific frames, i.e. without the extra cable stops, hole and routing for front derailleur.
I have a 3x8 converted to 1x8, and the trick is to put a 48 tooth big ring with a road cassette. That is even in gear size increases otherwise it wont work. The chainring being 48t holds the chain well as over half its diameter is biting the chain.
The only time I have a problem is if the chain is worn and slack, and I do a big jump in 8. The chain has fallen off once but overall you get a nice smooth shift transition.
I'm on a 22 pound (10 kilogram) mtb with 2.4" tires and it keeps up with and often beats skinny tire race bikes on road and then at the same speed can ride offroad and do massive jumps. I did this 6yrs ago and haven't looked back.