You've got me!
It's never happened to me before, I had them try 7 times, then it just became apparent they wanted to wait. And time wise it works out rather OK. Since space in the house is becoming premium, waiting would minimize gen 6's chance of becoming to old before bringing in generation 7.
But, I doubt that was why Frida and Simon were waiting. I'm but a lowly watcher from Dimension H, what do I know about sims thoughts and plans.