USD is also much less volatile than BTC, so easier to gage what the crypto is doing rather than what BTC is doing. Obviously trading between X and BTC you need to be aware of both BTC & USD, but otherwise, if you're investing from USD in the first place then that's going to be the most relevant comparison for you.
Example: I invest from GBP, fiat trade with EUR, crypto trade with BTC, but use USD as the reference... because the mid-term difference of EUR/GBP/USD is so minimal compared to cryptos.