Rationale behind vacuum instruments

IIRC, the FAA mandates a fully redundant electrical system (two
completely separate electric buses, each with its own alternator and
battery) before they'll allow you to run your primary gyros off
electric power. The added weight of such is often substantially more
than just a vacuum system alone.

Also, in almost every aircraft I've flown, Only the AG and DG run off
the vacuum system, while the Turn coordinator is electric. This way,
if either the vacuum or the electrical systems fail, you still have
enough of a partial panel to get you through a cloud layer, at the
very least.