what exactly caused the experience?

Well, first off, I'm aware that this to an outside observer seems sort of insane, or at the very least unreliable. You had to have been me in that exact moment to understand exactly what was going on, because it's mostly incommunicable. In many ways It did happen out of the blue. It was much quieter than you might expect. I had been reading up on Christianity pretty consistently throughout my life, at first because it was how I was raised, then because I was a philosophy major, and it's next to impossible to study philosophy without also studying Christianity. So I had "all the facts" but never any reason to believe them. After college, I continued reading about it. One day, I was reading the autobiography of St. Theresa of Calcutta on the patio of a coffee shop, and I saw a homeless man digging through the trash. And then, I had this moment of looking around me, and it was like...everything was different. All of a sudden, I had this almost tactile understanding of the truth of the Incarnation, and what exactly happened during the breaking of the bread. Everything just sort of clicked, and it was one of those things where it's so obvious in retrospect that I wondered why I had never seen it before. With that came the feeling that I was spiritually starving--a sort of panic at being keenly aware of just how separated I had been from God until that moment and the peril my soul was in every second I spent distancing myself from God--like I needed to get to Mass right this very instant. And as it happened, there was a church 5 minutes away with a Mass starting in 10 minutes. I know I was meant to go there in that way. I never would have planned to go to Mass and followed through. It had to be the impulse of a moment. So I was sort of moved/compelled there. God met me exactly where I was in exactly the way I needed to be met.

Why not?

Because God is present in the physical world and can be found here.