Population density is never the only reason that governments invest in huge, expensive infrastructure projects... there's always politics too:
British Columbia Highway 5
In the 1960s, the Merritt Board of Trade began lobbying the B.C. government for a new highway route to Hope, including a vehicle caravan that was staged eight times starting in 1963, over the abandoned Kettle Valley Railway grade, in order draw attention to the potential of this route. Surveying commenced in 1973, and in 1979 the first construction contract was issued for a 4.5 km (2.8 mi) section of highway between
Nicolum Creek and Peers Creek near Hope; however, work progressed slowly until 1984, when Premier
Bill Bennett announced that the project would be fast-tracked so it could be completed to coincide with
Expo 86. To ensure the project was completed on time, more than 10,000 workers were needed, and more than 1,000 pieces of heavy equipment worked non-stop every day during the summer of 1985.
en.wikipedia.org