Why is the CMOS logic voltage exactly 3.3V, why not 3V or 3.5V?
"Logic Families" (http://en.wikipedia.org/wiki/Logic_family) are typically built for compatibility and involve "standards". A particular family generally employs identical silicon level transistors to perform a wide range of functions. Designing a die component requires considerable engineering, so it makes sense to reuse them.
A "new" generation is typically designed with specific goals in mind, including, but not limited, to backward compatibility, lower power consumption, faster response time, higher bandwidth and smaller die area. In most cases 3.3V logic can interface with 5V logic, but consumes less power, is faster and has a smaller die. 3.3V is certainly not the lowest voltage CMOS logic family, 2.5V and 1.7V logic are quite common in high density, high-speed ICs like RAM and Processors.
I hope that helps answer your OP.
By lowering the power supply from 5V to 3.3V, switching power was reduced by almost 60 percent (power dissipation is proportional to the square of the supply voltage).