It's also a tradeoff with the cost of the NIC hardware --- look at how much bandwidth xDSL technologies have been able to squeeze into a single traditional twisted-pair phone line, but they use far more complex signal processing techniques than Ethernet.
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Signal integrity basically. Using more pairs means reducing the bandwidth needed on each pair, thus the cost of the cable.
It's also a tradeoff with the cost of the NIC hardware --- look at how much bandwidth xDSL technologies have been able to squeeze into a single traditional twisted-pair phone line, but they use far more complex signal processing techniques than Ethernet.