There's a trade-off between speed and memory use. Accessing and updating individual bits is slow, due to the underlying hardware architecture. MATLAB goes for a compromise that is good for most cases.
There is no 1-bit data type in MATLAB. If you really need to save memory as a top priority, you can store 8 bits to a byte using bitset and bitget operations on an array of integers.
EDIT 29 Nov 11
An update: I've just remembered bwpack and bwunpack in the Image Processing Toolbox. You could possibly make use of these.
