Time limit
Lotus had accompanied Lex for a long time, and had played a significant part in Lex's journey. Before anything else, Lotus was the first one to evolve Lex's body, making him stronger and more durable than others in his realm, which was a quality that continued throughout his realms.
There were times it cured poisons, not only in Lex but in others as well, absorbing excess energy to prevent Lex from blowing up, and countless other times it had been extremely useful.
It was kind of sad to see him go, but at the same time it obviously wouldn't live with Lex all its life either. At least since they were bonded, they could stay connected, and continue to benefit each other.