This is owing to the fact that we could associate any legal currency with cost averaging as a strategy of investment for either short or long term durations.
True but which other fiat has a global recognition and coverage if not dollar. It is nobody's fault that dollar like btc has dominance in fiat currency and moreover, it is usually pegged with usdt and when you want to buy coins that you want to be sure fees don't get above $1, you use usdt which bring back dollar into the business.
Most global transaction before btc has been with dollars and so it was natural for the usage as Dollar Cost Average. It is not only for buying of crypto but a general transaction for stocking and investment.