how and why does medea foreground marriage as an arena for tragedy?

I'm not entirely sure what you mean by "foreground" so I will just address the topic. If we think of tragedy (especially in the Greek days) as being stories detailing the collapse of 'great' men against the immovable forces of the universe (Fate, time, etc), then Medea differs only in that it poses the immovable force as a woman's dependence on the patriarchal structure reinforced by their place in marriages. She is certainly 'great' (not necc. morally good) - her cleverness, her magic, her depth of feeling - but is unable to rise above the system she's committed to. It's more than that Jason betrayed her love; it's also that he has made her an irrelevant woman because she is not his wife any longer. Despite her strength, she cannot just forget about this marriage, because her world forces her to be dependent on it. She damns herself through killing her kids. Of course, unlike many other tragedies, wicked Euripides made her the victor in an 11th hour save.