Could be as simple as Garcia saying, "Hell, I write in E, A, D, and G all the time, so I'm going to mix it up a bit for this one." Maybe exploring a new key brought out some different ideas for chords? Also, composers have known for a long time that different keys have different vibes to them. Remember when Phil changed Fire On The Mountain to G for a while, back in the early 2000's sometime? Didn't work too well IMO. Another example would be Dave Brubeck's "Take 5" which is in Eb minor (an easy tune to play, as jazz tunes go--one chord in the jam!). One would think it would make sense to move it up or down a half step into a "normal" key, but when you do, it just sounds wrong.
My band mashes up Dead and Phish songs (might be sacrelige around here!), and it's actually a pain in the ass sometimes because LOTS of Phish songs are in F, Bb, and Eb...can make it difficult to navigate our way into a Dead song or vice-versa. But I have always wondered why Trey & crew lean toward those keys more than most rock bands. Perhaps it's their jazz backgrounds.