It's not the junk food. It's not the lack of pacifiers. Why is it that something different requires parental blame of some sort?
Some kids are more curious than others, and sticking items in their mouth is a method of exploration. Others have sensory issues where a lot of things go in the mouth.
My almost 2 yo is super active, super curious, and always getting into stuff. So so so much more than our first. And, yes, I do feel like I'm dealing with a puppy at times. She's tried mud/dirt a number of times. She even stuck her tongue in the sand at the beach! (Twice(!) before she figured out it wasn't such a good idea.) This is her temperament. The number of things going in the mouth is tapering off, but I don't expect it to go totally away for a while.
My older daughter (4.5) has sensory issues and still puts things in her mouth sometimes.
They rarely eat fast food. Both have varied and relatively healthy diets. One used a pacifier obsessively (the one with sensory issues) and the other refused to take one.