EDIT: Also, I don?t think I need to mention that there is literally almost NO DIFFERENCE between Sun and Moon and Ultra Sun and Ultra Moon, and even though some people like me still bought and play them, a lot of people did not cave in and I?m sure it caused sales to falter.
I'll be the first to admit that USUM were pretty lacklustre third versions when compared to other gens, and are probably my least favorite by far, but to say there's no difference between the two is pretty unfair. There's a
big list of changes, such as introducing 4 entirely new Pokemon mid-generation (a feat that has never happened before), new Z-moves, Pokemon appearing in the background of the overworld, a new storyline, and so much more, none of which contributed to sales loss.
Third versions never sell as well as their original titles. Red, Blue and Green sold ~31.5 million, where Yellow sold ~14 million. Gold and Silver sold ~23 million, where Crystal sold ~6.4 million. Ruby and Sapphire sold ~16 million, where Emerald sold ~6.3 million. Diamond and Pearl sold ~18 million, where Platinum sold ~7 million. Black and White sold ~16.4 million, where Black 2 and White 2 sold ~8.5 million. Sun and Moon sold ~16 million, where Ultra Sun and Ultra Moon sold ~7.5 million. When you compare third version sales to original title sales, Black 2 and White 2 comes out on top selling ~54% compared to their originals, with USUM only falling behind at ~47%. Yellow comes in at ~47% as well, Emerald and Platinum are both tied for ~39%, and Crystal falls flat at a mere ~24%. Saying they didn't sell well isn't true at all, because they have the second highest percentage of third version Pokemon titles, tied with Pokemon Yellow. Keep in mind Gen VII is still new, so this number is going to continue to rise, and will have outsold Yellow percentage wise by the end of the financial year. Yes, maybe if they put more time into the game so it didn't feel like such an afterthought they would have sold better, but what they're sitting at currently isn't anything to dismiss.