It’s just George Lucas’ fast-paced writing, and the inherent under-characterization of everyone in a pulp-sci-fi action flick like Star Wars. Leia barely seems to care that her entire planet exploded. That doesn’t mean the audience is supposed to understand she’s cold or unemotional. Rather, the audience is supposed to just fill in the blanks and assume Leia was traumatized but there’s just no time to dwell on that during the emotional roller-coaster that is the events of A New Hope.