In some cases, videogame genres are named initially based on the games that popularized them. "Doom-clone" was a fairly common term before the more generic "FPS" was de-facto standardized. This despite the fact that Doom was not the first FPS. DOTA-clone was the standard name for that genre until LOL came along and invented the more neutral term "MOBA". Roguelike and Metroidvania are named for the games that popularized that particular style.
But there's more to it than that. They don't have more generic terms because they're not big enough genres to warrant having a term for them. Indeed, they're not necessarily "genres" at all; they're styles of gameplay.
For example, Metroid Prime is Metroidvania, but it's also an FPS. There have been Roguelike FPS games. These terms tend to describe very specific mechanics or ideas used in a game than generic terms like "RPG" or "platformer".
A Roguelike is a general combination of several elements: permadeath, significantly randomized content, etc. It describes games as diverse as Nethack and FTL: Faster than Light. The core gameplay of these games have pretty much nothing in common, but they have the general style of Roguelikes.
Metroidvania is a general combination of a single world, mobility-based navigation impairment (aka: the ability to reach areas is frequently determined by movement abilities rather than locks), frequent back-tracking, and a few other things. It's not as broad of a concept as Roguelike, but it still represents something beyond the standard genre conventions.