Because they're just fantastic in every way (well, OK, they have their flaws), and so much better than their previous incarnations (although the new stops DC Comics is pulling sure does make Brainy look hot...but Nura? Nnnnn, not so much.
And here's why they are an amazing pair. And this is before they became a couple.
And later on...
Their kid would be the bane of Cosmic Boy's life.