[bunderbunder]

Mechanical clocks need regular (probably daily) winding, and drift by quite a bit per day - as much as a few minutes.

The hordes of people you'd need to employ to maintain such a system might well have been expensive enough to make an installation like this the more economical option. And, given that pocket watches were a very expensive luxury item at the time, having lots of publicly visible clocks might have been a public service that could be justified on grounds such as enabling smoother commerce.