They won't for the same reason why a fruit seller doesn't sell car parts.
When you specialize in something, you try to focus your efforts there,and not spread across a dozen things.
True, but a lot of fruit sellers sell vegetables, too. ;-)
They might put up a separate brand, GIG for ex, to distribute Indie games too. As they already developed the whole service, that's just smart re-use of assets. My guess is that the target populations largely overlap, and although handling Indie developers is quite different from handling big publishers, it might not be a bad idea at all.
Come to think about it, it is a shame most Indie games have to resort to DRM-ridden services like Steam for lack of alternatives..
edit: Oh, as a sidenote, I would however agree with others who'd argue that GOG should be well on the rails first (that is, large enough marketshare, good customer support, enough reserves), before starting any spin-off.. So don't get mad with the GOG team for not doing this, maybe they would be more than happy in other circumstances.