Many reasons. For example, using negative voltage will reduce DC component in the wires, that will improve reliability over long lines, as now all you need is to sense the polarity of the signal, not the level. You'd also need high power reference voltage (for "1") wire going all over the board, which will be nasty polluted with uncorrelated switching noise, will sag in uncorellated way with respect to the "2" (Vcc wire) etc.

Well, this is stuff I read 40 years ago about tech nearly 30 years prior!