That's because they be cool and that's all that needs to be said. (Though my personal preference for analog-subtractive-plus-string-synth is the Korg Trident. Still, I have space for none of this.)
Never got my paws on one but always wanted to. Any VSTs that recreate it? I seem to recall seeing one recently. Might have been the new IK Multimedia Synth package but I think not. The interface I remember did closely resemble a Quadra IIRC. The IK thingee doesn't.