The reasons are what you might expect. She's a centrist pragmatist. She likes the military. She likes "free trade." She has clout and seniority. She does whatever the oil industry tells her to do but also says nice things about the environment. All the stuff that establishment media tends to like, Mary is definitely those things.
The funny thing about looking at elections this way is if Bill Cassidy were the 20 year incumbent and Mary the challenger, the T-P would be writing this exact same article about him. Notice the T-P says nothing about the issues that differentiate these candidates and would affect most voters. Mary favors an increase in the minimum wage. Cassidy does not. Mary supports the ACA. Cassidy does not. During a debate this week, Bill Cassidy said raising the retirement age to 70 "won't hurt anybody." (I.. sort of... transcribed.. or translated.. that debate in its entirety here.) Mary does not want to raise the retirement age.
The Times-Picayune doesn't mention these things because their approach to politics ignores the possibility that politics is really about anyone outside of the money club. They've chanced upon an endorsement of the slightly less awful candidate in this case. But the only reason they have is because it's all about maintaining the establishment. To institutions like the T-P, politics really doesn't have any other purpose.