The talk show legend explained her decision to Jimmy Kimmel Thursday ... sharing that her upcoming special about prescription weight loss drugs played a major factor in her decision to leave the company she's been with for years as an official ambassador.

As Oprah put it ... this weight loss medication TV special she's working on was very important to her -- so she wanted to shed any appearance of impropriety by being involved.

Specifically ... O says she didn't want people to think she was just promoting a special about weight loss meds and assume she was making money off of it by being attached to WW -- which is now in the business of prescribing those same medications, like Ozempic.

News of Oprah's WW exit dropped two weeks ago -- when she announced that she was donating all her shares in the company to the National Museum of African American History and Culture. Of course, she's also recently admitted she's on weight loss drugs herself.

Now that she's taken the step of leaving WW behind, she told Kimmel nobody could accuse her of doing all this for profit ... 'cause there's no money to be made at this point. She also says the WeightWatchers crew took the news of her bowing out pretty hard.

In a separate portion of this interview, Oprah shares why she embraced the miracle drugs -- saying her weight struggles over the years were tied to her genetics by a doc, and she went so far as to say that "obesity is a disease." She says she's lost 40 lbs. on the drugs.