Just from the perspective of Metroid as a series of "games", what attracts me to them is that they have a little bit of everything.1 I mean this not in a jack-of-all-trades sense, but more of a queen-of-all-trades sense. Its particular combination of sci-fi horror aesthetics, expressive player controls, actiony combat, exploratory map design, and ludonarrative deftness is something many games try emulate, but rarely do they hit all cylinders, let alone aim for them. Just going by Officially Sanctioned Metroid Products, I'd say Nintendo has hit that mark at least 5 times with the series, which is an incredible batting average (and the list more than doubles if you consider fanwork (and have the patience for it)).
-
Rhetorical exaggeration. Please understand.
Like, don't get me wrong. There are plenty of "metroidvanias" out there that are holistically great works in their own respects, but they don't quite hit the same notes. Hollow Knight's atmosphere is great in its dismality, and its exploratory design is second-to-none, but I found the movement comparatively unexpressive and the paucity of major upgrades unable to support the sheer size of the map. Similarly, games with Lore these days are dime a dozen, but few are able to match Metroid Prime's ability to use Lore to tell a developing story, instead of having it be a substitute for a story (original release only, screw the hackneyed revisions they made).
There's also the whole deal with Samus as a character. Sure, she has a cool suit and they always try to make her look conventionally attractive, but there's a bit more to it than that. The games and ancillary material provoke questions of the self, of whether her true persona is the one inside or outside the armor, and how her life traumas have transformed both.
◀️ Read right to left! ◀️ Manga Alert! ◀️ 
Image taken from the Metroid Database, pages 5 and 6.
In the canon, it's nothing especially deep, per se, but having imprinted itself on me at a formative age it's hard not to read a lot more into it or to see myself in her (hence my incredibly cringe-core self-insert OC).
tl;dr - when I was a young child, a Japanese entertainment company stuffed some brainworms in my ear. I have been under their thrall ever since.
There's a lot more angles I could approach this question from, but I hope this answer suffices. Thanks for asking!