It's kind of hard to justify a WAM for an annual contract. Then there's the problem with operator abuse of expensive equipment. Here in Ohio 16 cuts per year would be bush-hogging, because with only mowing a couple times per month WAMs get bogged down with that much growth.
But to answer your question WAMs are more productive or they couldn't sell them. They only have a road speed of about 25 mph.
I've been using WAMs for years. They have advantages but not in every situation.