Because he's a creep

Besson's second wife was actress and director Maïwenn Le Besco, whom he started dating when he was 32 and she was 15 after having met 3 years earlier.[43] They married in late 1992 when Le Besco, 16, was pregnant with their daughter Shanna

Le Besco later claimed that their relationship inspired Besson's film Léon (1994), where the plot involved the emotional relationship between an adult man and a 12-year-old girl (played by then 12-year-old Natalie Portman).[43] Their marriage ended in 1997, when Besson became involved with actress Milla Jovovich, then 19, during the production of The Fifth Element

Edit: And the multiple rape and sexual harrasment claims

He has a long history of dating / marrying very young women. His first wife (the one you referred to as getting left) was 15 when they officially started dating. He first met he at the age of 12. She was pregnant by 16.

There's also the weirdness of how his movies portray age gap relationships. The professional in particular is often criticized for how it both sexualized a young girl & portrayed her as the instigator. Natalie Portman has talked about how this movie brought her fame and long lasting trauma. Plus the original cut was much more problematic & the studio cut quite a bit before it was released -- so bussons original vision was even more pedophilic. Plus it is supposedly inspired by his real life romance with a 12 year old.

Here's a link to a essay that goes a bit more in depth: https://faroutmagazine.co.uk/luc-besson-leon-the-professional-truth

Edited to add: there are also the many rape allegations from various women who have worked with him over the years.