Why no 24 Bit Software Defined Radios?
Take a look at the equation for minimum detectable signal here:

http://en.wikipedia.org/wiki/Minimum_detectable_signal

Calculating the noise in a 20MHz bandwidth shows that it will be around 73dB.  Meaning that the noise floor in 20MHz will be around -101dB (or -96 or less in a practical sense).  Thus, the higher the bandwidth, the lower the dynamic range will be.  24 Bit is only useful in bandwidths around 1KHz.