Here are some reasons in no particular order: 

This simplest reason would be that it is a spouse of one of the other chosen survivors.

Then a diversity argument can be made. You can say that it would be desirable to have people from all different backgrounds in your bunker. It could even be the outcome of some sort of lottery. 

Another reason could be that housekeeping skills would be valuable, especially when you are all stuck in a bunker.  

I hope this is helpful.
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