In my shitty opinion, the game lacks a solid target audience.
Grandmas won't buy this game for their grandkids. I worked big box store electronics retail when the first game came out and it was a hard sell specifically because it's Mario but with guns and grenades. It's a weird combo that works (in sparks, blasting two enemies mid air as mario ALWAYS kicks ass) but parents don't want to hear a game that is described as a "tactical shooter."
Real young kids might like Mario, but again, tactical shooter isn't really a popular genre for that age group. There are certainly some levels that I was like "they want kids to play this?" cause they were wild. And Rabbids really aren't a popular series. I had to look up what else they were in and since 2017 it was just some educational mobile games which also don't scream Rabbid appeal to me.
It doesn't appeal to adults because I was also initially turned off by the prospect of Mayro + Ubisoft Minions With Ears but I like the Rabbid cast cause they're defined characters. The first game had adult humor but mostly small jokes in item descriptions. And this gem:

Sparks has more adult jokes upfront and atmospherically it's more coherent, comfy and less absurdist. I bought the first game literally cause of Kirkhope and damn he goes hard on both.
It doesn't appeal to tactical game players because this has been a banner year for tacticals. We saw this game, Triangle Strategy, Tactics Ogre, Live a live, Front Mission, Hard West 2, and Im sure a bunch of indie titles. At the end of the year, I just don't think this was on as many peoples' radars that would pick it up otherwise.
It doesn't appeal to a younger generation of gamers because it's not Fortnite. That's not being reductive. Grant Kirkhope himself told a story in an interview during AGDQ i think 2016 during the Banjo Kazooie run. He son was playing through Rare Replay on the xbox and Grant sat with him while his son played through all the Banjo Kazooie games (without even telling his son he did the music). And Grant said his son like the third one the best cause you could build. His son liked BK but preferred Minecraft cause he could play with his friends. A $60 game with 30 hours of single player game won't win against the appeal of free with your friends with custom modes.
So that leaves me, the 20 to 30 something who played the first one, to buy it. The first game was actually discounted when it dropped and I really think they had low expectations cause it was so weird. But man that combat got every gear in my head turning. It got a lot of buzz that translated into awards because it was unique. But you can't make a sequel and have it be as fresh. Playing sparks i thought a lot about my coworker who sits with his kid when he games sometimes. Something the kid could laugh at and the adult would appreciate the music and help his kid through the tough parts. That would be the ideal audience to me.
And yeah, I was kind of upset too when I made it to the end and I said "wow, this really was a great game. I'll remember this," but I knew it hadn't done well financially (and it must have been expensive. they got a whole ass orchestra for the game and the credits have tons of people). But I think I'm fine with the feeling of, "wow the second game was a cult classic. It didn't do well but the people who played it loved it," instead of, "yeah the third game was bad, but the 4th was just awful". It deserves so much more but I appreciated it and some people here do too. And that's fine. But I might actually cry because my
favorite track in the game has less than 1000 views on youtube. The comfiest track of the comfiest world deserves better.
Since I'm sperging, there's an ancient greek concept of the 4 temperaments based on the four humors. Each one is a (flawed) personality type associated with a certain bodily fluid and element. It's still a trope used today. But each one also has an associated season. And with the 4 worlds in Sparks being based on the seasons, the Wardens of those worlds also have a matching personality from the 4 temperaments, with the last warden being the most capable with balanced humors. It could be accidental because archetypes / universal unconscious, but I'll believe it was on purpose and that the game is more clever than people think. They went so hard on the Wardens, giving them all back stories and unique art for what are essentially generic quest givers. Just such love put into everything really.