Less tech savvy relatives and friends are not on Snapchat/Instagram. Plus, Instagram feels more like a picture book than a musings diary. Basically, Instagram/Snapchat are less 'communicative' (I mean, they have their uses, of course).

To answer your question, probably laziness. I've got all those contacts on FB and would take a decent effort to connect on other platforms. Secondary to that, I'm sure several of my family members are FB-only.