Probably because consciousness requires some kind of underlying mental processes, which take time to achieve. I can only guess that it's similar to an instance of a computer program such as a web browser, i.e., it isn't a "thing" itself but is built out of a multitude of underlying calculations on a physical computer.

This also implies to me that consciousness, not being a physical thing itself, comes and goes within the brain, with the sleep cycle or just through lack of attention. The only interaction between one instance of consciousness and the following one would then be via memories.