I think there are two aspects to this. First, Húrin was the mightiest human warrior of all time. Before he was captured, he slew so many of Morgoth's minions that his battle-axe "melted" under the strain. Húrin was not the greatest hero among the Secondborn; his son Túrin and Beren Erchamion certainly outrank him in accomplishments, and perhaps Tuor and others as well. Morgoth, however, by the time of the Battle of Unnumbered Tears, had lost much of his subtlety. So the Great Enemy could recognize martial might, but little more, in his enemies. Having taken such a great hero as Húrin alive, Morgoth probably felt he had captured his greatest human foe; and who would be better to torment than that?
Second, and alluded to above, Morgoth's sadism toward Húrin (and likewise toward Maedhros hundreds of years before) was emblematic of how much Morgoth's goals had decayed. Mightiest of the Ainur, Melkor had originally, before the First Music, wanted to create a world according to his own design. Then he wanted to warp Arda to his will, but when this proved impossible, he was reduced to despoiling and stealing the great works of others. As this happened, his power and puissance dissipated. He lost the ability to take any form but that of "a dark lord, tall and terrible," nor could he recover from mere bodily wounds inflicted on him by Fingolfin. By the end of the Elder Days, he was reduced to pointlessly tormenting humans, so far had his powers and aspirations fallen.
