A friend of mine asked why vacuum (suction) is used to drive the gyros in small aircraft and not high pressure air.
A good question.
anawat pointed out that some types use pressure instead of vacuum:
Not all GA air instrument are vacuum. Later beechcraft: Baron and Bonanza use pressurized air instead of vacuum.
...either way the same type of gyros and pumps are used so it does not matter in principle. I think that wiggy and LTCTerry are correct - less chance of contaminating the gyro if the pump is downstream as per the vacuum arrangement, therefore less filtration, simpler and less chance of ruining a gyro when the pump fails. Backed up by the fact those types using pressure have dry pumps. If they used wet pumps then oil would get to the gyros.