https://screenrant.com/why-frasier-lilith-got-divorced-after-cheers/

Frasier and Lilith reunite on Freddy's birthday in the Frasier reboot episode 7 (https://screenrant.com/frasier-episode-7-recap/), but the show doesn't reveal what exactly went wrong between them.

She cheated on him with one of her research partners during one of the latter seasons of Cheers and then moved with that research partner into a biosphere. Later, she left the biosphere and tried to reconcile with Frasier (who she caught in bed with Rebecca upon her return). Obviously, it didn't work out.