Because the midi spec does not define a way to send negative controller values.
It's 0-127 or 0-255 for hi res/pitchbend. In other words, there is no midi device that will do what you are talking about.
If you want to affect a parameter both above and below a zero point, you have to set up the receiving device to handle it that way. If the synth/sampler you are using doesn't give you a way to route control messages and adjust the offset between the internal synth parameter and the external midi value, you can't do it.
