I have no skin in the Motown game, in fact I didn't really like it. But in the interest of expanding on this, I think you have mis-characterized its success. Number of performances isn't the only metric. Leaving out the lackluster return to Broadway for three weeks, which didn't have the momentum of the original behind it, the grosses were enviable. They didn't have a single week under 800K and only 20 weeks were less than 1M (all in the 800K and 900K's). This out of roughly 96 weeks, and the average weekly gross was 1.2M if my math is correct. On top of that, it never fizzled out. Grosses were consistent and strong right until the end. Wasn't the rumor that they were forced out because Harvey needed a theatre for Finding Neverland? I remember being shocked when Motown announced closing because nothing in their sales patterns indicated they were going in that direction. You can pretty much tell when a show is going to announce closing (unless they have some angel pumping money in) and Motown wasn't like that.
It also had very strong numbers on tour, doing business near or just below the blockbusters. There is no question that this not very good musical was very popular and did very strong business.