I could see big fans of the setting wanting the tarot deck. But not all setting fans are currently playing, and not all players/GMs are such big setting fans. For that matter, most games will not have direct interactions with the sort of major background character apt to be shown in the tarot, so it is pretty easy for a group to skip this until the impact shows up in later books.
It is an interesting and different product, and kudos to all involved for making it happen. But to me it seems a bit niche for setting fans. Granted that SR has accumulated a lot of setting fans over the various versions. (And King of Sartar was a niche publication for fans of the Glarantha setting and was pretty huge for that setting; hopefully this can be as inspiring)
Edited to fix the name of the Gloranthan publication. King of Dragon Pass was an awesome, quirky, computer game, but what I meant to name was King of Sartar, a book that revealed certain setting secrets and hinted at many others.