Given the noise level in the city, a bell would only cover a few blocks. Also, this solves a different problem: having accurate clocks. Unless you want to stat

Given the noise level in the city, a bell would only cover a few blocks. Also, this solves a different problem: having accurate clocks. Unless you want to station an attentant under each clock to reset it when the bell rings. The assumption is that there's a lot of drift in the clocks of that time so frequent re-syncs are necessary.