Why are there almost no bikes with a single chainring? 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.