My reasons are really tangled, the cultural, physical and personally symbolic aspects going together.
With my history of being very disliked (and that's a merciful understatement) and feeling that I'm scraping the bottom of the barrel of life, I've always felt drawn to scavengers. Even before finding out about other furs, I had vague fursona-like ideas, and it was a jackal at that time.
After finding out about the fandom, I gave this matter the more thought. Looking through various species, I stumbled upon the striped hyena, at that time practically unknown to me. And this was it.
The striped hyena doesn't like to over-exert itself (to say the least... call it lazy), but will gladly pounce on an opportunity. It's less social than the spotted one. Somewhat hysterical, when faced with an opposition is equally likely to either feign dead, or go berserk. But otherwise can be pretty chill, even friendly, and all the grumpiness is more of a habit and doesn't need to result in anything bad. Well, that's me...
Aside from meat (more or less fresh...), striped hyenas actually like fruits a lot. To the point of causing damage to plantations of melons, watermelons and dates. Well, if I had access to either, it might have ended up with causing some damage too.
On more symbolic levels, my love for death metal (and other kinds of "angry" music) contributed a lot. Obviously, the hyena is a scavenger, accused even of robbing graves. Quite fitting. As is my skewed sense of humor and tendency to laugh maniacally at things which others don't necessarily understand. Also, in the folklores of the countries where it lives, the striped hyena is associated with witchraft and magic. Not bad for someone with esoteric interests.
As funny as it seems, there can be some physical traits even. I can stomach almost anything (within reason!) and any gastric problems are extreme rarity for me, usually a sign of overbearing stress rather than any bad food. There have really been cases when I was unaffected by things which caused (mild) poisoning in others. And when designing the fursona, my own mug turned out to be quite good match for hyena's heavy, blocky muzzle. And (according to Wikipedia), the striped hyena needs more water than most other carnivores. I need rather lot of water too, to the point where I was suspected of having diabetes or thyroid dysfunction, but nothing came up in tests.
The looks did play a role, of course. The lush, long coat with contrasting stripes. A long mane, easy to re-imagine as long hair (I did have mine almost butt-length back in the days; currently shortened for purely practical reasons but maybe I'll revert...). And those ears, for some reason I love things with big, pointy ears. Overall, if you watch without prejudices about "filthy carrion-eater" etc, this is a really beautiful animal.
All in all, it's a match like no other for me, and despite some experiments with alternate characters, in my heart of hearts I'm a striped yeen going strong since something like 14 years at this point! To paraphrase my favorite author Bruno Schulz (the original quote was about the dog): "Please don't get me wrong. It was a hyena – undoubtedly, but in a human shape. The hyenish quality is an internal one and it can manifest itself equally in human as well as animal forms."