“Because Hashem changed it to 210 years because of Hard Labor.
We were supposed to have 400 years of regular labor, but if you make it Much Harder, you can Condescense the time.
It came out to 210 years!“
How about saying it like this that Hashem gave us hard labor to take us out after 210 years (according to one way of learning, according to the other way it started from Yitzchak)