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My guess is loss of coal, oil, and gas jobs. What year is the data from?

Yup, the core population isn't leaving, that is just odumass's job killing policys that caused those statistics!

Wyoming has booms that draw a lot of workers from out of state. Many people have no idea about the climates the state offers. While I was working there we lost a lot of people due to the wind blowing, add some cold and they were really miserable.They couldnt handle it.
Guys that loved it their wives hated it.
Wyoming is not for everybody, which is why it is a great state still.

It's hard to believe that more people are leaving Wyoming than Illinois!!

Lots of kids leave after school to look for jobs.

I am still here!

So....14% of Wyoming....that like....6 people then right?

https://www.census.gov/newsroom/press-releases/2017/estimates-idaho.html

I agree ~!

Percent change doesn't tell the complete story. Absolute change also needs to be shown. I've made plenty of maps showing both and they can tell different stories. Illinois is a much larger state in absolute people numbers so there were more people who left IL than WY. With that said, however, WY is the lowest populated state, AK passed it x number of years ago (can't remember for sure, after 2000 census?). So, a 15% decline out of 579K is a pretty big deal, unless the past few years there has been a influx into the state for the various reasons already posted. The Census Bureau loves their yearly percentage change stuff!

A story of the two Dakotas. South Dakota reached its first population peak in 1930. It took a big hit with the depression and the dirty '30s and didn't recover/surpass its 1930 number until 1990. I think North Dakota had a similar path and for a number of decades (1950s through 1980s, maybe 1990s)the two states stayed fairly close to each other, about 10-20,000 people difference in the 600,000s, with South Dakota slightly ahead. Then, with the 2000 census, South Dakota started to pull away from its "sister" state and now its estimated (2017) to be over 100,000 more than ND (869,666 to 755,393). Cities such as Fargo and Bismarck in ND have grown but Sioux Falls, followed by Rapid City have grown faster. South Dakota also has a larger Native American population living mostly on the various reservations in the state. If you look at the Census data for the "top" poorest counties in the US, using per capita income, SD usually has 5 out of the top 10, all reservation counties. But looking at the median age in those counties, it typically is low (such as between 20 and 30). With a lot of young people still in their reproductive prime, the reservations, even with all their social issues, will continue to grow...