What made the difference? What did the
Israelites have the the Egyptians didn't?

During the time that God gave them before
that plague struck, there was safety and
immunity from disease and death for those
who wanted it and would received it.

It was for the Israelites and Gentiles because
there was a "mixed multitude" that went out
with the children of Israel by the creation of
the Passover. Which is a type of the atoning
work of Jesus Christ, the Sacrifice of the
spotless Lamb of God.