JB writes:

The reason is that single engine aircraft, of course, have only one
engine. A multiengine aircraft has 2 or more engines. You would
either have to guess which engine to attach the monitor to, or install
a bank of switches to jump back and forth between monitoring the
various engines. I read somewhere that both configurations were
tried. Planes crashed because, inevitably, the monitor was always
attached to the engine that always worked fine (ha ha Murphy's Law) or
in the case of multiple switches, planes crashed because pilots became
disoriented flipping the switches or because they didn't keep both
hands on the wheel.

Engineers gave up so an engine monitor is no longer found on
multiengine aircraft.

What about multiple monitors?