Not to mention the fact that some people think of WiFi and cellular data as "things that give me access to the internet". The understanding of what a private IP address is and why it can't be reached from a cellular connection is just not there.

Others want to control their IoT when they're not at home or not in WiFi range (they may not even notice the latter). You can do it with a VPN, or perhaps port forwarding if you're lucky enough to have access to your router and no carrier-grade NAT, but that's even harder to set up.

3d printer users are more sophisticated than most, but I can imagine some artsy types owning them, as well as the kind of people who are very comfortable with a drill, soldering iron and a jackhammer, but who treat a computer as "that God-damned machine I need to use to buy the parts I need."