I think there is a ton of reasons. Most significantly that there is not much modern ICs that can handle the battery voltage and even less MCUs. I'd bet that CPU core of the chipset Jolla uses operates on something between 1V and 1.8V. Most IO logic levels are most likely 1.8V. 5V circuits are quite obsolete these days and even 3.3V starts to get rare. Battery voltage (around 3.6V in this case) is almost never used as IO voltage. Most likely the i2c logic level of OH is 1.8V.

Raspberry pi uses 3.3V. If you're going to use it with raspberry pi, I'd recommend regulating the voltage to that.