Too many indie platformers in the backlog. I see no reason to buy a full priced game, when you can get even better ones for half or one quarter of Wonder Boy's price.
Next purchases for that genre will be Inside, Shantae Half-Genie Hero, Snake Pass and Yooka Laylee

It deserves the sales. It's one of the best games of the year. Much of the credit goes to Westone for the original design but Lizardcube deserves a lot of love for the care and attention that went in to the remake.

Check Amazon sales rankings for the digital code:

https://www.amazon.com/Sonic-Mania-PS4-Digital-Code/dp/B073WH2JKL/ref=sr_1_2?s=videogames&ie=UTF8&qid=1505138267&sr=1-2&keywords=sonic%2Bmania&th=1

As of this post, Switch version was highest:

PC: 2514

Switch: 192

PS4: 623

X1: 1338

This suggests that the Switch version was the best-selling one.

Reap the rewards for releasing near launch.

Still, the original article had other interesting tidbits from other developers:
https://www.gamasutra.com/view/news/305284/Rise_of_the_Nindies_Indie_devs_discuss_porting_to_the_Switch.php

The comparably limited pool of great grames is obviously a sales advantage for those titles, but yeah, saying Switch owners will buy anything is pretty hyperbolic. Given the choice between paying for a shitty game and just not buying anything, most are going to choose the latter.

Other systems may have a bigger pool of games, but they also have a way bigger pool of consumers.

We are talking like 5 million vs. 65 million, or whater Switch/PS4 is.

Edit: but wait this is combined. So like 100+ million, or considering probably on PC like 200+ million potential buyers on other systems.

Seems easy for a game nobody has ever heard of. Hard to find such a game in actually big libraries.

A combination of the game's art style looking quite nice on the portable screen and the general need for games on the system itself. Although the indie releases have increased greatly over the last few weeks, when Wonder Boy first came out I almost bought it just because I was desperate for games on my Switch. The library seems to be picking up at a decent pace, however. 

The game is $30 on the AU PSN store. Why would anyone buy this when you've got sales with games like Bloodborne for $11? The Rayman Legends + Rayman Origins pack is currently $12. Bastion and Rainbow Moon are $6 each. Surely they don't expect to compete at that price.

And? Zelda is $60 on e-shop, and also Mario Kart, Splatoon and many others are twice the price of Wonder Boy. And yet it sold more on Switch