The same reason we don't measure everything in nanometers, because units are a tool and we should use the one that makes the most sense at the time. Bits shouldn't replace Sats but it is a unit just as sats are. So when we have a number like you said 9180 sats, we can reduce that to 91.80 bits or you could say 91bits and 80 sats. Which makes it more readable because there is less clutter around it.