Because he started as a peak-stage palace realm cultivator—and a sword cultivator at that—siring a child proved to be difficult. Years passed without success, those years turned into decades, which turned into centuries, and eventually millennia without a child.
He went through partner after partner, outliving them all, yet none bore him a child. He had almost given up—until 1,200 years ago, when he met his current partner.
Unlike the others, he hadn't chosen her out of a pragmatic need to fulfill his promise. By then, he had resigned himself to the idea that keeping his word might be impossible. With his lifespan's limit slowly approaching, he had decided he might as well attempt the domain realm breakthrough. If he survived, the considerable lifespan he gained would give him more time to try again, even though the difficulty of siring a child would likely increase tremendously.
But he held off—because of her.