Mass shootings get the most attention even its rare. No one really cares about daily gun violence. The fact that you can get killed by a random person going on a rampage is shocking. Since kids, families etc. get killed it's a lot relatable for people.
I think top reason for mass shootings is American individualist culture. Look at western European countries that have as or even more individualist cultures, access to firearms. Yet shootings rarely occur. Main reason being that they have a lot more closely-knit societies. That allows for direct/indirect social controls.
Lack of social conformity, violent culture, way too easy access to firearms, media giving too much attention to shooters. All combination of these.