The jack. Also the iPhone 6 was bigger and the rounded edges and corners introduced a bend/crack hazard, so in a back pocket, sitting down, there was enough leverage across the larger size to introduce force that bends it in the middle, cracking the screen.

So, the iPhone 5 appeals to people who want a small, durable, fully featured smart phone, with a normal headphone jack. The iPhone 6 and up all have bigger screens, but 6 is the last version with a headphone jack, and it is definitely less computationally powerful and measurably slower than everything after it.