Nationalism is a modern invention and Catalonia's status within the Crown of Aragon is irrelevant here. The Crown was abolished in the early 1700s, a full century before Catalan nationalism emerges.

As to why Catalan managed to survive and reemerge as a major language in the 20th century [1], to start with, it was much more widely spoken than Aragonese. And so it had better chances to resist the encroaching usage of Castilian that I talk about in this [earlier answer] (https://www.reddit.com/r/AskHistorians/comments/6iqusl/why_did_castilian_become_the_default_spanish/).

Aragon had smaller population than Catalonia, the usage of Aragonese was in decline even before the two Iberian crowns united in the late 1400s and it never truly developed a literary tradition. Catalan had way more speakers and enjoyed a higher status to begin with, being a prestige language of royal chronicles and Troubadour poetry. Granted, those things didn't mean much in the 1700s or the early 1800s. But once it survived into the mid-1800s, the early Catalan regionalists in the mid-1800s had a living language to work with and a literary tradition they could look up to/build upon.

We'll need an expert in the 19th and 20th Spain to weigh in as to why Aragon did not develop the kind of separatist aspirations that Catalonia did but I'd imagine it might have had something to do with Catalonia being an early hotspot of Spain's industrial revolution and having a thriving middle class as opposed to Aragon, the most rapidly depopulating region in the country. But this is way beyond the period I'm comfortable talking about as an historian.

[1] The Aragonese language does live on. But it is now only spoken by several thousand people in the high valleys in the Pyrenees.