What happened to Sprint's nationwide WiMAX plan?

WiMAX wasn't the same thing as WiFi, despite the similar name. It was a wireless broadband technology used for a mobile network, not a plan to fill the country with ordinary WiFi hotspots.

Sprint backed WiMAX for its early 4G service, but LTE became the technology the wider mobile industry adopted. Sprint moved to LTE, and its WiMAX network shut down in 2015. Sprint was later bought by T-Mobile.

Coverage was part of the problem too. Higher frequency signals generally have a harder time reaching long distances and getting through buildings than lower frequency signals. That can mean more cell sites are needed to cover the same area, especially indoors.

There's some disagreement over how much of the problem was WiMAX itself versus the frequencies available for the network. Either way, making a network genuinely nationwide takes a lot of infrastructure.

LTE didn't solve the coverage problem by making towers unnecessary. It's still a cellular network, so it needs sites serving different areas. Mobile coverage got much better, but that isn't the same as having a signal everywhere in the country.