How did the Oakland Athletics become a struggling small-market team? How did they fall from grace so hard?

https://www.reddit.com/r/baseball/comments/141yh10/how_did_the_oakland_athletics_become_a_struggling/

On paper the Oakland Athletics should be one of the league's biggest name teams. They're the most successful American league team not named the Yankees, having won the second most pennants in the AL, and 5th most in all of baseball. They have the 3rd most World Series wins behind the Yankees and Cardinals. They've constantly had a good record for a good portion of their history in Oakland. So you would think they would be one of the most popular storied franchises out there. But as we all know they're currently struggling big time on the field, their attendance numbers are way down, and they're about to leave Oakland.

How and why did the Athletics fall so hard?