I can't particularly offer you a reason there because I can't get myself to romance her with an elf either, tbh.
I looove her with my Adaar, it's such a great romance, they're so cute together, it's delightful and amazing, but I've run two Lavellan's and neither one went for Sera.
It seems like most of the people here who have romanced her with elves aren't particularly big on the Dalish or their religion or they are playing an atheistic character or one that has an arc that turns to doubt/distancing themselves from the Dalish, so they don't run into any problems, but when I play Dalish I like to be Dalish as heck.
Maybe if Crim sees this she can speak to it? I know she does the elfy-elf too and she did it with Sera.