The others over here have already done a good job of explaining why the focus is voltage over current. I would like to add another reason to the list, in terms of signals and data:
In terms of signals and data, the voltage of a signal represents its strength whereas the current represents its speed. While analyzing a system, we focus on voltage because we want to compare a signal's entering strength with its exiting strength. When we are talking about individual devices, voltage across each end of the device will almost always be different but the current magnitudes on both sides will be the same. Thus it makes more sense to focus on voltage rather than on current while explaining circuits in books.
However designers do not focus only on the voltage. They compute each and every thing in their circuit and consider voltage, current, resistance, capacitance, and inductance - everything is considered.
I hope that helped.
Humza.