Has he said why he won't? Seems like a departure from the 'I'll bore you to death with hours of irrelevant superficial answers' strategy he's driving in the US.
It's a bit of an odd turn. I'm not sure if they're up to some shady shit and are panicking. Or perhaps they simply don't really know how to handle the situation.
I have heard that the penalties for giving "incorrect" (i.e. lying) answers to Parliament are more severe if he does it than if one of his deputies does. Not sure what or why there would be that difference, but based on Facebook's track record, expecting them to be honest is a stretch.