Why isn't the litre a SI base unit?

The Liter is a derived unit, thus not strictly part of the Systeme Internationale.

The SI units for volume are measured in cubic metersand, for all practical purposes, the cubic centimeter and the milliliter are the same, and are generally used interchangably.

But, strictly speaking, "liters" measure CAPACITY, not volume. In other words, liters are used to measure the capacity of a container to hold a material, as distinguished from the volume of the container itself.

http://physics.nist.gov/cuu/Units/