Why is this poem a metaphysical poem?

Metaphysical poets were known for their use of surprising images and for putting together unlikely ideas. Dr. Samuel Johnson, who coined the term "metaphysical" to describe them, notes that in these poets

the most heterogeneous ideas are yoked by violence together; nature and art are ransacked for illustrations, comparisons, and allusions . . .

In "To His Coy Mistress," we can see the metaphysical influence in the extreme use of hyperbole (exaggeration) that the speaker employs to try to persuade his beloved to go to bed with him. He says that if he had enough time,

An hundred years should go to praise
Thine eyes, and on thy forehead gaze;
Two hundred to adore each breast,
But thirty thousand to the rest . . .

His point is that they don't have centuries for the niceties. He can't spend two hundred years flattering the beauty of one of her breasts.

The speaker drives his point about time home with an unusual metaphor, comparing time to a "wing'd chariot hurrying near." Soldiers in ancient times drove chariots in battle, so this is an image of time as a frightening force chasing down the lovers and threatening to kill them. Time is coming at them so fast that it is as if the chariot were flying on wings. Marvell could have had no experience of airplanes in the seventeenth century, but he describes time, if we were to use a modern idiom, as a fighter plane swooping at supersonic speed toward them. That is an odd image for a love poem.

The use of this kind of heightened exaggeration and the unusual metaphors are metaphysical qualities in this poem.