I rarely see Maytag portables for sale. Perhaps they were more expensive when new, as compared to, say, their Kenmore and GE counterparts. If many portable dishwasher buyers were price-sensitive, like say a newlywed couple, it would make sense that Maytag might lose sales to less expensive competitors.
However, Kitchenaid seems to have sold a ton of portables, based on how many of those I've seen for sale. And Kitchenaid products were always expensive. So that theory may not hold up.