Welcome Seswilson.
I do not know the answer, but I suspect it's an accident. The release of Rattle and Hum created a backlash in the USA and U2 chose to tone down the "Americanness" of the tour - which included not touring America. Heartland is easily the most American song, so I suspect they didn't play it live for that reason. Then they went on the ten year hiatus from all things not weird. By the time they did a show where Heartland would have fit in, IE Elevation, it just wasn't a fan favorite and didn't earn a spot.