Gold. Lots of gold.

According to the National Park Service, the Alaskan population of white people in 1890 was about 4300 people. (Population numbers on native peoples are fuzzy at best.) The gold rush kicked off around 1896, and the population of Alaska exploded as people moved north seeking to stake their claims and strike it rich. Depending on the source you use, the 1900 white population was anywhere from 30,000 to 100,000. As the population expanded, so did the infrastructure to support it, including military garrisons and trade routes. That's a lot of money headed north, and although a lot of folks went back south after the Gold Rush, many of them stayed and other industries grew. Even today, Alaska is a major seafood exporter, not to mention petroleum resources.

Labrador, although further south, never really had the major draw of a Gold Rush type event to boost the population. There was some fishing and fur trading, but there really wasn't much non-native settlers permanently living in the area until the 1940s and 50s when large iron ore deposits where found in the region, which prompted some population growth. Military interests during WWII led to some additional growth, but even still there just wasn't the major industrial draw that Alaska has enjoyed.