Some of this is the second-childhood effect --- Walt Longmire (Robert Taylor) is Matt Dillon (James Arness) reincarnated (minus Miss Kitty), updated a little and relocated from 19th century Kansas into the 21st and northwest to the mythical Absaroka County, Wyoming, where the scenery is more compelling.
The plots --- which have lumbered less as the series progressed --- are neither more nor less implausible than other TV fare, but the aforementioned scenery is better.
And, wow, the native population is treated respectfully. Favorite characters include Zahn McClarnon (Standing Rock Sioux) as tetchy tribal police officer Mathias (top) and Lou Diamond Phillips (with a bit of Cherokee ancestry but more Filipino) as Henry Standing Bear. Many other native actors find work here and their culture is treated well and features large.
The West, of course, is America's mythic landscape and its mythologized characters, the black-and-white heroes of my generation (Longmire's audience base tips toward the over-50 crowd).
Much of that mythology is nonsense --- and a good deal of the carefully cultivated culture of the Old West still is. Decimation and marginalization of indigenous peoples along with slavery are twin horrors that will demand national penance into the foreseeable future.
But golly, Longmire sure is fun --- in a bloody sort of way.