Why are breadboards laid out in strips of five?
As kenny surmised, the main reason was to accommodate 0.600" DIP packages.  Back when microprocessors first started appearing, many of them came in 40-pin DIP packages like this Z80 show below.  Nowadays most microcontrollers that come in DIP packages, even those with 28-pins and above, are available in the narrower 0.300" form factor.



Even some 7400 series logic devices which had many pins (such as the 24-pin 74150 mux and 74154 demux) came in 0.600" DIP packages.  These wide packages only allow for two connections on either side of the device.  

Makers of small DIP modules, like the Gabotronics Xprotolab mini-scope shown below, are now making use of this extra width.  Note that only one pin is available on either side of the module to connect to other parts of the circuit.