As others have stated, Detroit was eliminated for sluggish growth, not low population. Having said that, I'm definitely in their target demographic (35 yrs old, BSCE and MSEE, 10 years software development experience and team leadership experience), and I think that their 'has the right labor pool' filter is a bit off. I'd guess that Amazon is looking to rein in their labor costs a bit, and often a large city with some tech (but not a ton) can provide a very good labor pool at a reasonable cost. They got rid of a lot of places I'd be willing to re-locate to in that round (I can't stand huge cities), but perhaps I'm alone in my desire to live in a place that has low cost of living, reasonable traffic, and abundant housing+land.

PS - I actually currently live and work near Grand Rapids. I have 11 acres that didn't cost me a fortune, and I love it. GR would probably have also failed their 'amenities' filter, which I think is also wrong - any place with broadway, concerts, sports teams, good restaurants/breweries/bars etc. is probably good enough for most young people.