Rationale behind vacuum instruments

Can someone tell me why vacuum power is popular for certain instruments? I
cannot see any special reliability of a vacuum pump as opposed to an electric
motor. The only reason I can think of is historical, in that many small
aircraft have traditionally not had on-board electrical systems beyond what
was required by the engines themselves. However, a vacuum pump seems no more
reliable to me than an alternator. What are the reasons behind it all? I see
lots of descriptions of how the instruments work, but none that explain or
justify the choice of vacuum over electrical power. It also seems that vacuum
is subject to partial failures, whereas an electrical failure is usually much
more obvious.

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