Just because they are called Greeds, doesn't mean they represent actual greed.
No, they pretty clearly represent the Buddhist idea of all desire, for basically anything, as intrinsically evil and bad. Did you think all of the dialog from Kougami and Ankh about LOOK AT THESE DISGUSTING HUMANS AND THEIR ENDLESS GREED FOR
THINGS was incidental? Especially that one speech by Kougami about how
THE FIRST THING A BABY DOES IN THIS WORLD IS START DEMANDING THINGS!!!
These are
not Christian attitudes and not Christian characters. These characters are not discussing any Western or Christian understanding of the idea of greed or desire. They aren't even discussing anything like a Christian notion of sin, because nobody in this show has given a damn about any concept of God or the divine yet.
Kobayashi is using OOOs as an excuse to tackle one of the central conceits of Buddhist belief, one that most Japanese pay lip service to most of their lives but don't take seriously until they're old and dying. That's a pretty ballsy thing to do, all things considered, and not just because OOOs is a kid's show meant to sell toys. If you go after Buddhism you're going after something your audience really believes in.
Japanese media about Christian concepts, on the other hand, tends to be kind of... well, dickless. Impotent. I really can't think of a better way to express it. The overwhelming majority of Japanese people are not Christian and do not give a crap about what Christians think. You can make fun of Christian ideas all day and not run the risk of offending anyone, so there's no reason not to treat Christian ideas in cartoonish ways or play 23 skidoo with actual belief (as per Evangelion).
If OOOs brought in the 7 Deadly Sins you'd most like get a goofy and
very shallow treatment of the idea, even moreso than what we're getting with Buddhist demons now. I think maybe a grand total of 1-2% of the Japanese population will actually self-identify as Christian. There's no reason to take that stuff seriously, as either creator or audience, so it won't lead to good writing. Especially from someone like Kobayashi, who these days is clearly about examining very Japanese things for her very Japanese audience.
though if we got some aspects of buddhism maybe we'll see other show up... 
