Why did the Jews not Leave during the Plague of Darkness they Egyptians where standing still and could not see them Leave?
1 - The time of redemption has not yet arrived.
2 - It was a much bigger miracle to leave when the Egyptians were watching, and could not stop them.
3 - The promise to Avraham Avinu of leaving with a Rechush Gadol had not been fulfilled yet.
4 - They were busy burying the Jews that died.