Because infrared radiation is absorbed by water vapor in the earth's atmosphere, to see it you have to get above the troposphere, which contains most of the water vapor.
(infrared radiation; is absorbed; by water vapor in the earth's atmosphere; to see it you have to get above the troposphere) (you; to get; above the troposphere) (the troposphere; contains; most of the water vapor)
infrared radiation you the troposphere
is absorbed to get contains
by water vapor in the earth's atmosphere to see it you have to get above the troposphere
above the troposphere
most of the water vapor