Personally I think it's cause she gained favoritism through elements among both the new and the old viewers. If you're a hardcore fan prior to the reboot, you know about the mystery of Della Duck and that fact that yes, Donald actually does have a twin sister in the Disney canon of things so obviously anyone would be interested. This series though did a very good job at introducing her character, they gave us little hints about her whereabouts and what she was like all throughout season one without us seeing her in person that it made the viewer tell themselves "god, I wish we could just see her for ourselves." and when they gained that hype, it definitely paid off for when she was officially integrated into the show. it was the kind of hype where we'd think "yes, I really am interested in seeing where this is gonna go."