In terms of setting the mood for a space fantasy, a name like ‘Leia Organa’ is very Space Princess. Luke Skywalker, on the other hand, is quite down to earth while still being a little space-ish. This makes sense when he’s the audience avatar in ANH. When he’s some super magical unicorn space wizard in TFA, putting ‘Luke’ in the crawl still feels to me like he’s a farmboy who hasn’t gotten his Jedi name yet, hence the brother and sister speak.
It also cuts down on the number of terms being name-dropped in the crawl and the potential confusion for first-time viewers of Leia Organa also being a Skywalker also being a Solo.