That's indeed a problem there. I was there for a walk on a cold and windy day. After walking for a while, we wanted to go back, and it turned out horrible, because back was against the wind, and there is nowhere to hide from the wind. Sure, you will not leave civilisation here, but trees would help.
They probably don't plant any because when they finally do decide to build a new residential quarter, they'd also have to fight against tree savers. And probably there's a regulation that for each tree you fell, you need to plant a new one. So better not plant too many trees without good reason...
I think that the open plain is what makes the Tempelhof Field what it is. If you want trees, Hasenheide is right next door, or you can go to any of the other forested areas. But for an really open place, there is no such place in the city. I agree that some tree cover at the sides could help against the traffic noise but the openness is what makes this place special.
They probably don't plant any because when they finally do decide to build a new residential quarter, they'd also have to fight against tree savers. And probably there's a regulation that for each tree you fell, you need to plant a new one. So better not plant too many trees without good reason...