They're not trying to accomplish the same thing musically. Generally speaking, it would seem that U2 makes their music from a place of extroversion, while Radiohead makes their music from a place of introverison. Now, this isn't a 100% foolproof thesis, but I think it holds true most of the time. U2 wants to scream their music from the rooftops and they want every single person on the planet to be singing along with them. Radiohead doesn't neccessarily have anything against that, but surely they have a big soft spot for the kid in the back corner of the classroom lost in a pair of headphones, or the person awake a 3AM sitting on a couch lost in a pair of headphones while their family sleeps, or the person sitting on a long airplane flight in the night lost in a pair of headphones.
Like I said before, I know that this isn't foolproof...I'm sure there are people in classroom corners, on couches, or on airplanes, lost in a pair of headphones listening to U2 too....but I'm talking in generalizations to make a point.
Radiohead seeks to make music that will seep through your skin into your bloodstream, flow through your body like a virus, weigh you down, make you implode under the weight of its brilliance. At the conclusion of a song or album, you might sit motionless in reaction to what you just heard.
U2 seeks to make music that will attack the exterior of your body, make you wanna jump up and down, pump your fists in the air, shake your head back and forth, scream, just explode, because its brilliance is too much to contain inside of you. At the conclusion of a song or album, you might be unable to stop running/jumping/screaming/singing, unable to shake the excitement and energy you feel, in reaction to what you just heard.
It's different kinds of brilliance.
You can't attempt to squeeze orange juice out of an orange and an apple and say 'The orange is better at producing orange juice'.