The AAA game's industry has really ruined people's perspective of what is a slam-dunk success. It wasn't *that* long ago that hitting a million copies sold was the end-all milestone of the outrageously successful.
1.5 million of an indie title with this niche of an appeal is an astronomical success. It's basically unheard of in the indie space. Hell, Shovel Knight has outsold some Nintendo games, a company that has a several decade legacy of fans and marketing power.
I don't think there really is ground to wonder why it hasn't sold more, I think we should continue being flabbergasted it sold as much as it did. 5-10 million is such an absurd number, that's not a number many highly marketed AAA vast-appeal games reach, a game like Shovel Knight is successful at 100,000 copies. 1.5 million is just unbelievable.
Like...not to rain on your comment either, just more of a riff of your comment that I just don't get the mindspace or swayed perspective that this isn't an unbelievable success and you don't get why it didn't sell more. Lots of AAA games market hard, hit the door, and fall flat on their face below a million. The overwhelming majority of indie games try their hardest and sell 10,000 or way less. This is an insane success!
1.5 million for an indie game that's never been bargain-basement discounted is a blockbuster success. Even moreso when you keep in mind that NES-style graphic games usually bomb.
I guess that I just think that console / Steam games should sell more, both in general to people at large (outside of mobile stuff, which does manage to reach millions of people) and specifically within the console / Steam bubble that we mostly still occupy together, especially when it comes to well loved titles like Shovel Knight that are like a perfect storm of praise and attention.
Heck, even the biggest AAA games are still relatively niche in terms of sales compared to number of movie tickets sold for a blockbuster movie for instance, which as someone who truly believes that there is a gaming experience for everyone - but who frequently speaks with people who've never tried
any game ever (or even heard about someone like Mario...), this sometimes frustrates me. It also makes me wonder why this remains the case when it feels like gaming in general has become more accepted and talked about than ever before.
I guess what I'm getting at is that sales numbers for games always feel kind of low for me compared to the fervor with which games get talked about, the size of the games industry and growing games coverage industry, and the amount of attention and coverage some titles get.
So yes, within our usual confines Shovel Knight is indeed a great succes that I applaud. But within the larger world of entertainment its sales for me reaffirm (A) that most console / Steam games, even universally praised ones that are priced relatively low, are not really picked up by the large majority of players for some reason (i.e. None of them become games that over half of a console's owners own, which is something that in the NES days could still happen) and also (B) that the larger public is still not really interested in games in general or in console / PC games specifically even when they play on their smartphones or go watch similarly themed movies etc. I don't really have a better reply than that, sorry.
Edit: the Mega Man 2 example mentioned below is a similar case where its one of those games that pretty much everyone talks about with some reverence to this day, while it is being re-released frequently as well, but where the total sales are "only" 1.5+ million in total. It feels as if a larger perecentage of gamers would have bought it on some console by now, but apparently not, which again makes me wonder why (except perhaps for most of the current sales coming from second hand copies that don't count towards that total).