[satyrnein]

Some potential "costs" of voice/video: it's (often) exclusive to one person, it's immediate, and it's easy to expose your own emotional state. In an emotionally fraught, developing situation, perhaps your nieces want to take it slow, check what their friends think, not let on that they're upset, etc.

This is all just speculation, I'm not really a texter, but I do find it interesting when limitations might turn out to be features.