SI has definitely been trending in this direction for at least 15 years, probably more. This is certainly not the first instance of a cover story that has a nominal sports hook but is really devoted to political advocacy for standard-issue liberal positions.
I'm not sure what led to the magazine's demise, but one irrefutable fact is that it just doesn't have the same quality writers as it had in its glory days. Writers like Tex Maule and Mark Kram made great dramas out of the stories they covered, with the athletes as characters. I remember Kram describing a punishing combination from Muhammad Ali as "dark, magnetic Goya." Frank Deford could be (and still can be) too flowery for my tastes, but at his best he had a lyricism and wit that is sorely lacking in the magazine now.
I wonder who their audience is at this point, with so much competition coming from web and cable outlets like ESPN. If it is an older audience that remembers the magazine's heyday, how much longer can they be expected to hang around while the magazine's decline becomes an inconvenient truth?