After Legacy, it's been stated pretty clearly that Sophie will always be unmatchable. Unless Oralie steps forward herself, Sophie is stuck as a bad match to whomever she loves.
Fitz is not okay being a bad match. That could corrupt his family's reputation even more than it already is, and he would not be okay with that.
Keefe on the other hand, couldn't care less if he was a bad match. In fact, he might even want to be one so he can make his father look bad. He's always been rebellious. He'd love to brand his father as The Man Who's Son Is A Bad Mach.
Sophie obviously doesn't care about being a bad match, or she would've turned Oralie in already.
It's also been shown that one of the cutest couples in the series is a bad match: Juline and Kesler. They're honestly one of my favorite couples, just because they choose to be happy despite all the hate thrown their way.
Plus, if Sophie—the most powerful elf alive—was a bad match, then there's a good chance others will start to accept bad matches.
In conclusion . . .
Sophie and Keefe getting married could solve one of the biggest problems in the Elvin world.
