Vacuum Gyro Question

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?
The reason I'm wondering is because about 2" of suction is
sufficient to power a turn needle, and while attitude gyros require
more, about 5", either of those amounts is still much less than
standard atmospheric pressure, meaning that the absolute pressure
inside the instrument is still close to ambient pressure.

Note other than seeing one on an instrument panel, I've never
actually taken apart or looked at the inside of any of these instruments.

In other words, what am I missing and what don't I understand? Thanks!