Most local business is completely based on local customers. Exceptions such as tourist industry operations do exist but are a small percentage of total local business.
Internet local search/directory has not yet matched the convenience, efficiency, and trust of paper yellow pages, asking the neighbour, etc. How many people when the water heater bursts think to go online to locate a plumber at 2AM?
Plumbers, etc. certainly can afford to pay more than most small businesses but unless you can prove that their local customer base looks for plumbers online why would they be interested when they know for sure that base read the local paper and the phone book and they are likely booked for the next 2-weeks anyway?
When selling ads/sites you must be able to produce some numbers, some value. I see very few hard statistics of local search beyond the tourism, community, and entertainment verticals.
Why should the chicken switch to live birth while the egg method still works fine? The plumber, like the hen, does not need local search to get results. Targeting such a demographic requires a hook whether it be future-proofing, branding, helpful content, etc. to get beyond mere need.