"Ska" is much more specific than "indie." It's like you're not understanding why I hate squash, but like a ton of other vegetables.
Ah, but therein lies the semantics. I would say music as a whole would be like the vegetables, whereas ska is squash, there are many kinds of squash, butternut, acorn, hubbard, etc. I always classify indie the same way as ska, it has a sort of sound to it, yes there's different styles, but pretty much I can go, "alright, this is probably something indie, but what
kind of indie?" just from hearing it, same as I can go "alright, this is ska, but what
kind of ska is it?"
I have a lot of very good ska bands, but there are also a lot of shitty ska bands that came out when ska was popular the second time, and I generally dislike some of the bigger name ska bands just because its not the style that I enjoy. I'd be willing to toss out a bunch of ska to anyone that was curious about it, and if you don't like it, well, that's your opinion, I didn't like a lot of bands for the longest time because of what I
thought they were, and how they sounded. Years later, I listened to them, or offshoots of them, and realized that I did like the music after all, and that just because I was a ska-kid, that didn't mean I couldn't like metal, rockabilly, or indie.
I think the big thing is not to just flat out say "oh I hate ska" but rather take it from a "well, I dislike this band, but this other band of the same genre is pretty good" stance.