Older chips like the 8080 had multiple supplies (+5, -5, +12), but chips with a single supply were much preferred. They could have generated a higher voltage on-chip, since the 8086 did have a charge pump to generate the negative substrate bias voltage. But distributing an additional voltage rail would be inconvenient when there was just one layer of metal wiring. So it was probably better to use the bootstrap driver (which only used one additional transistor).