Well, unless the explaination is simply "I don't want her to." and as a parent, isn't it ok to just say no to things you don't want to do?
It's not like this is a granny who's not getting to see her DGD, she gets to spend a lot of time with her. She does get 'quality time' because she gets the DGD in the day while she's awake, alert, they can do stuff that takes a bit of time etc.
And what is the OP going to do with her answer? Try to work a way round it? I'm uncomfortable with explaining parenting decisions beyond "because we do/don't want X" to extended family because then you've given them something to argue round.
The OP has made it clear she's happy to have her DGD overnight, she's made it clear not only would she do it as a favour to the DIL and her DS, but because she wants to have her DGD (so it doesn't need to be for a reason like they have a wedding to go to etc), the parents have declined the offer politely.
Trying to find ways to get them to change their minds isn't going to end well, and pushing will probably end up with the OP getting less time with her DGD not more.