Jim Carriere wrote:
This is one of those questions where there must be a simple answer. This
is probably the forum to get the best explanation.

Why aren't vacuum driven gyro instruments driven by pressure instead?

Well, they are. They are driven by a pressure differential between the
input and output.

As for why we lower the output pressure rather than raising the input
pressure...I'm guessing that a venturi in the airflow is
easier/better/more efficient than trying to use some sort of ram air
device to raise it.