I think people just want a decent car, not a luxury car. A $20k car from the last five years is perfectly sufficient. Uber Black is overkill for them, and the price is around 2.5x of UberX.
Also, in a lot of cases, I find that the customer gets upgraded anyway (at least in off-peak times).
Many, many times I have ordered a regular "black car", and been picked up by a big SUV (Escalade or Suburban). Likewise, a lot of times I have ordered Uber X and been picked up by a luxury car and sometimes an SUV (Audi Q5, etc).
True--while most Uber vehicles are black, it's not a requirement according to a few drivers I've queried. The BLACK category indicates a luxury vehicle for which a BMW 5 would certainly qualify.
Many, many times I have ordered a regular "black car", and been picked up by a big SUV (Escalade or Suburban). Likewise, a lot of times I have ordered Uber X and been picked up by a luxury car and sometimes an SUV (Audi Q5, etc).