A flip-phone is just a phone. You can text with it, but it’s cumbersome, and there’s not much else to do with it.
A smartphone is much more than that — the entire Internet is there for you to explore, for good or (usually) for bad.
Most people with smartphones waste hours of their day on social media, texting and Internet browsing, which they could be using for important things they’re neglecting, like Torah, davening, tehillim, mitzvos, chesed, sleep, exercise, getting your work done, paying attention to your children, etc. The same goes with people with constant access to the internet due to laptops, etc.
LU, about the untznius images everywhere on the internet, fortunately there is a partial solution for those of us who need the internet for work — you can find browser ad-ons (like Wizimage for Chrome) that automatically block all images.