I don't think we're ever given a reason for why Bellatrix is chosen in the play, but...
Think about it logically. We know that Voldemort wouldn't have had the child out of love or passion - this is Voldemort we're talking about, a man who only cares about achieving immortality. From that perspective, the only reason he could want to have a child is to help him towards that goal (which almost worked in Harry Potter and the Cursed Child) - therefore, who better to choose as the mother than his most loyal and devoted supporter?
Bellatrix LeStrange is devoted to both him and his cause to the point where she would do literally anything for him. Narcissa Malfoy, on the other hand, wasn't even a Death Eater herself - she was simply the wife of one of Voldemort's more useful, influential supporters. Logically, why risk losing the Malfoys as an ally (I can't imagine Lucius being happy about Voldemort sleeping with his wife) when Bellatrix can do the same job?