I am guessing it is because once it is observed you know the exact position of the particle. Because superpositions are a collection of possible wavefunctions you have now eliminated all the others and are left with the single wavefunction that you know describes the particles behavior.
P.S. You really shouldn't try to bring any kind of normal logic to quantum mechanics. A good example what you are talking about is how the the spectrum of a double slit experiment completely breaks down if a detector is placed on one of the slits. That's just how stuff works at this level.