"...In many locations, the number of signups are only in the hundreds, and it cost millions of dollars to build up the networks. Add to this the fact that wired high-speed Internet like cable and DSL is available for the same price, and there isn't much reason for people to sign up anymore..."
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They typically have not put themselves in an individual consumer's shoes, offering up too little bandwidth for too many $$$.
Various fixed line & cellular competitors offer "worldwide" hotspot service-- so why would an individual either, ditch a current carrier, or additionally purchase service limited to their immediate municipal area(which most leave to shop & commute to work / school on a daily basis) unless if the speed or reach(say inside subway tunnels) was state of the art.

Whoever is putting up municipal wifi needs to:

1. Go the wholesale & independent agent routes as well: sell space to isp's, cell carriers & utility companies worldwide-- so that for instance, a gas/electricy/cellular/satellite provider can then bundle internet service... any local ISP can offer a more valuable product...
Factor in additionally mutual territorial access/roam agreements, and pretty soon the coverage area extends way beyond the city to perhaps blanket the country & even other countries: a company like say, Virgin would love to be able to bundle a package encompassing national / international cellular & internet...

2. wire up subway tunnels & major buildings.

3. utilize the latest & fastest technology / equipment, no matter how expensive initially-- to make yourself attractive to both individual and corporate consumers.

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