Art Spiegelman is different than other artists whose work is considered at the height of the comics craft. He didn’t have a daily newspaper strip like Bill Watterson or Charles Shulz. Neither was he a pure writer like Neil Gaiman or Alan Moore, whose respective works have solidified their places in the comics (as opposed to comix) pantheon.
In the comix tradition, Spiegelman’s work is not innately commercial. Rather than fitting into an industry with a broad distribution and devoted customer base like traditional comic books or daily strips, Spiegelman’s art goes deep and tells stories, more often for adults, that are so compelling that their merit cannot be ignored. They aren’t really fantastical or cute. For his commitment to his vision and craftsmanship, it is fitting that his work be on exhibit in a museum of fine art.