A few reasons:

• Stranded wire is more expensive to make.
• At a given wire gauge, stranded is going to be larger than solid wire (it's the cross-sectional area of conductor that counts and there are going to be some air gaps between conductors with stranded). This could make a big difference if you have several cables in a limited space like an electrical box.
• The main advantage of stranded is that it's more flexible. You generally don't need this in home wiring because it's all put in place once and hidden behind walls/floors/ceilings.
• When you screw a solid wire into a switch or receptacle, you can tell if it's secure. I could see individual strands coming loose as you fold wires back into an electrical box.

More information in Wikipedia: http://en.wikipedia.org/wiki/Wire#Solid_versus_stranded

As long as all the equipment is listed for the use, there's no problem using stranded wire. The UL White Book says that screw terminals and pressure plate terminals can be used with both solid and stranded wire.

UL White Book 2013

Receptacles(RTDV)

Receptacles for Plugs and Attachment Plugs (RTRT)

Terminals
Terminals of the wire-binding screw, setscrew, or screw-actuated backwired clamping types are suitable for use with both solid and stranded building wires.

It says switches are solid only, however, you can use terminal connectors such as fork connectors.

Switches (WFXV)

Snap Switches (WJQR)

Terminals
Terminals of the wire-binding screw, setscrew, or screw-actuated backwired clamping types are suitable for use with solid building wires unless otherwise indicated either on the device or in the installation instructions.

Terminals of a flush snap switch are permitted for use with Listed field-installed crimped-on wire connectors or an assembly, if so identified by the manufacturer.

A flush snap switch may also be provided with conductor leads with factory-installed crimped-on connectors. Such connectors may be either attached to the flush snap switch terminal or are provided with the flush snap switch in the smallest unit shipping container and are suitable for use with the terminal of the flush snap switch.

Wire connectors (including twist-on wire connectors) can be used for both solid and stranded wire sizes between 30-10 AWG, in combinations listed by the manufacturer. Unless they are clearly marked to be used with one type or the other.

Wire Connectors (ZMKQ)

Wire Connectors and Soldering Lugs (ZMVV)

Wire stranding -- Unless clearly marked "Solid," "SOL," "Stranded" or "STR" for a given wire size, wire range or wire combination, conductors in the range 30-10 AWG are both solid and stranded, and 8 AWG and larger are for stranded wire only.

Ideal Industries has a handy PDF, that lists all the allowable wire combinations that can be used with their various connectors. For example, the Ideal 74B® (their basic yellow wire-nut) can be used with 1 - 4 #14 solid or stranded conductors.