In the viva your examiners have freedom to ask about everything in your thesis. They can also ask about anything that isn’t in there.
You have to have boundaries for your research. You can’t do everything, but you’ll probably think about a lot more than you finally include in your thesis. Your answer can simply be, “That was out of the scope of my research,” but that might not be answer enough for your examiners.
So explore the why with them. Why didn’t you do it? Don’t take the question as a criticism; questions explore. Give your examiners your reasons – lack of time, lack of interest, lack of resources, lack of information, deliberate choice to focus on something else… Whatever it is, but be real. Don’t bluff. Be honest.
There might be interesting, exciting things that you’ve not done, but you must have done something good to be in the viva.