Because of two reasons- Boyle’s law and Armstrong's Line
Boyle’s law states that as the pressure of a gas decreases, its volume increases proportionally. So if the U2 suffers a decompression or if the pilot ejects from the plane at 70000 ft. (which is the standard operating altitude for the U-2 Dragon Lady), the sudden drop in pressure would cause the gases inside his/her body to expand rapidly and he/she would ‘burst’ from the inside out.
(yes, her. There have been 8 female pilots till date to fly his insane aircraft)
Also, Armstrong's Line states that at 63000 ft., the atmospheric pressure is so low that conventional liquids boil at 98 degrees Fahrenheit (37 C). So if a pilot were to be exposed to the elements at U2’s operating altitude, his/her bodily fluids such as saliva, tears and sweat would literally be at its boiling point. The pressure suit is designed to protect the pilot in case of such an occurrence by providing counter-pressure within the suit.