Like Plinky said she became sick. In the first timeline there was a plague that had no cure and Diana wasn't healing the people like she used to, she was only helping the nobility. But now we know that's because she was losing her healing powers. Also the reason Hes made the soap business in this timeline is to help quell the plague.
*Novel Spoilers*
I don’t really know how the manhwa will do it but in a couple chapters Hestia talks a little bit about what she did after Cael died in the first timeline. Based on Hestia’s monologue it was actually suicide. Hestia gave one last chance to Diana to heal her but Hestia got kicked out of the church so she just fully gave up on healing since she took it as a sign to die so she just let the sickness take over her body since it was the least painful way to die from the other options she was looking at. Afterwards, she believed that once she died she will wake up back in her original world but that didn’t happen.
When Hestia was explaining, or Hesplaining to Kael her theory of why he no longer has feelings for Diana, being that he loved her so much that he let her go. You saw that look on Kael's face after? That was a look of a man completely defeated. He's tired of explaining to her that he doesn't like what's her face. The man was resigned. "Well. Hestia will be hestia. Just let her have this."
Also, anyone remembers the reason Hes died before her regression? Was she sick? Was it suicide? Why does she not believe she'll make it past this year?