I’m just assuming that you’re either a troll anon or someone who’s only watched the movies and never read the books, but I’ll answer anyway.
First of all, you can read my feelings about Éowyn here. That’s really a very brief summary of why she’s awesome, but it’s better than nothing.
Second, I’d like to remind you if you’ve read the books (and if you haven’t, I’d like to let you know) that ALL of Arwen’s scenes in the movies were added by PJ & Co. Literally ALL of them. We only see her twice (I think?) in the books. Once briefly in Rivendell when Frodo notices her talking to Aragorn, and second time at her wedding when she has a short speaking part. That’s it.
So gee… I wonder how it’s possible to like someone who’s actually active in the story, affects the plot and has displayed all sorts of vulnerable and strong sides of her more than someone who appeared twice briefly in the whole book series. Go figure. (Seriously, what you’re saying is like “I don’t understand how it’s even possible to like Arya Stark better than Elinor Tyrell!” It’s not just possible, dear anon, it’s the more logical thing to do.)
I’m not saying you’re wrong to like Arwen more than Éowyn (though I’ll certainly think that’s a very strange opinion) but it’s really quite possible to like an important character who had a legitimate effect on the plot more than one who shortly appeared only a couple of times.