Why do they call the 21st parallel the "Tropic of Cancer?"?

Answer 1

During the June Solstice when it was first named, the sun was in the Cancer constellation, though this isn't the case anymore. The Tropic of Cancer is the northernmost latitude where the sun will appear directly overhead at noon, which happens on the June solstice. The other tropic, the Tropic of Capricorn, is the southernmost equivalent (and of course, the sun will appear directly overhead during the winter solstice).

Answer 2

At the time of the summer solstice, the Sun is directly above the 23rd (not 21st) parallel; at the same time it enters the zodiacal sign of Cancer. Similarly, at the time of the winter solstice the Sun is at the 23rd parallel below the equator, and enters the sign of Capricorn; thus, that line is called the Tropic of Capricorn.

Answer 3

We got 'Cancer' at 23 1/2 degrees N.

We got 'Capricorn' at 23 1/2 degrees S.
One famous Canadian calls it "Crappycorn" - fact.