Because that is how they wanted her character to be.
Helga never died of anything.
Hey Arnold - 1996 Arnold Betrays Iggy Helga and the Nanny 3-10 was released on: USA: 19 October 1998
Hey Arnold - 1996 Arnold's Room Helga vs- Big Patty 3-4 was released on: USA: 9 September 1998
Hey Arnold - 1996 Helga's Show The Flood 3-15 was released on: USA: 25 November 1998
Hey Arnold - 1996 Helga's Locket Sid and Germs 5-7 was released on: USA: 19 April 2000
Hey Arnold - 1996 Helga Sleepwalks Fighting Families 4-13 was released on: USA: 23 October 1999
They dance in the April Fool's episode where Helga is pretending to be blind. I don't know if they dance in any other episodes though :)
Hey Arnold - 1996 Helga's Masquerade Mr- Green Runs 5-2 was released on: USA: 11 March 2000
Hey Arnold - 1996 Helga's Love Potion Gerald's Secret 2-2 was released on: USA: 28 September 1997
Arnold's sister in the TV show "Hey Arnold!" is named Helga Pataki. She is known for her tough exterior and her secret crush on Arnold, which she often expresses through her sarcastic remarks and bullying. Helga's character adds depth to the show, showcasing themes of love, insecurity, and the complexities of childhood relationships.
In the series finale of "Hey Arnold!", titled "The Jungle Movie," Helga and Arnold do not get married. However, the film does suggest a romantic connection between them, with Helga confessing her feelings to Arnold. The story leaves their future open-ended, allowing fans to imagine what might happen next in their relationship.
Helga confesses her love to Arnold's answering machine in the episode entitled "Helga Blabs it All."