The Super Mario Bros. series is one of the single most important franchise in video game history. Maybe the most important in terms of influence, sales, and sustained creative output. The most recent new entry in the series, Super Mario Odyssey, managed to feel as lively and inventive as Super Mario Bros. did more than 30 years prior. That's some impressive staying power.
Given the central role Mario has played in Nintendo's business for nearly four decades, the company has been pretty diligent about keeping his games—especially his core adventures—in circulation. You can currenly play multiple versions of the original Super Mario Bros. on Switch, 3DS, Wii U, and the NES Classic Edition, and the same holds true for its predecessor, the non-super Mario Bros. And that makes the obscurity of Mario Clash, a game designed 100% as a follow-up to Mario Bros. by many of the same creative staff, all the more unusual.
Sure, Mario Clash was tied to the Virtual Boy hardware, which Nintendo still seems bashful about acknowledging, but this is a legitimate core Mario game! Developed internally at Nintendo! And yet, all they've ever done with it is make passing references in WarioWare. Such a pity.
Like a lot of Virtual Boy titles, Mario Clash turns out to be pretty interesting, lots of fun, and somehow just a little underbaked. One of these days, Nintendo needs to put together a collection of Virtual Boy games like this and Teleroboxer, like one of those old Game & Watch Collections for Game Boy. The 3DS ship has sailed (just peep those numbers of the Bowser's Inside Story remake). Maybe a Virtual Boy Classic Edition mini-console?
No, I have no idea how that would work. Let that be Nintendo's problem.
In the meantime, you owe it to yourself to track down games like Mario Clash, one way or another. They're too interesting (and too closely tied to some of gaming's most pivotal works) to languish in Virtual obscurity forever.