Because (conceptually) the smallest unit of data on the JVM stack is 32 bits. So there is no way to increase the size of the stack with just 8 bits.
http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-2.html#jvms-2.6.2
At any point in time, an operand stack has an associated depth, where a value of type long or double contributes two units to the depth and a value of any other type contributes one unit.