You're not wrong that Ryland and Mark feel very similar. But I can't blame the Weir for wanting to put a character with a similar archetype in a more epic story with a new format.

He could obviously have made it a sequel and just had the character be Mark. And, honestly, I wouldn't be surprised to learn that that was the case in an early draft/outline. But doing so would have some unique consequences on the story and challenges to writing the story he wanted to. He would have to worry about precedent and character development from The Martian carrying forward into PHM, but making it accessible to those who didn't read The Martian. He'd have to justify why Mark has gotten himself in this new situation after barely surviving the last. Having a character we already know would fundamentally change the nature of Ryland's backstory reveals and the character flaws we learn about in this story (I don't know how those elements would work with the Mark we know). And making Mark into franchise character would be an odd choice in a story with such a different flavor of worldbuilding and with the consequences of events in this story.

Using a character that is similar to one people loved from your other work may seem like a creative cop out, but I see it more as an iterative development on the archetype he is interested in. And I like the archetype. People write anthology-esque stories with similar themes, settings, subjects all the time. I have no qualms with a writer having a recurring character archetype that they tweak between entirely different stories, especially when they are written to be clever and endearing, but still flawed people. It's a matter of personal taste, but I resonate with that character type and find them compelling and fun to read. I don't mind that it's not a serialized story/character when the stories being told are each self-contained.