Two cases: Dense urban areas and sparse lightly-trafficked areas. In lightly-trafficked areas, you're absolutely right about the cost limitation.

In the dense urban case, where "elevated pedestrian walkway" means "getting people away from cars", it means decimating foot traffic to stores and restaurants, ceding the area to automobiles. Generally regarded as horrible for dense urban environments.You see this more in urban planning a few decades back, and it tended to not help the surrounding area. If you look at the places where people want to live and travel to, you don't see elevated walkways. So it's fallen out of favor, with more emphasis on getting speeding cars away from dense areas.

The exceptions are things like the NYC Skyline, which is basically a pedestrian freeway between and through areas already dominated by pedestrian foot traffic and relatively light and slow vehicular traffic.