Why did I freeze when I was groped on the subway?

This summer I stayed in a big city while staying with my older sister for the summer. I was waitressing at a restaurant for the summer and had to wear a short skirt uniform for the job. I was riding on the subway at the rush hour to go to work for the evening and the train was totally packed. I got pushed up against people in front of me and people were crushed behind me. A few minutes after the train left the station for the longest part of the ride I felt something brush up against my under my skirt. At first you think its just accidental but then I felt it happen again and it definitely was a hand and instead of brushing lightly it was now like caressing my ! I tried to move away but it seemed that all I did was make it worse. I sort of just choked a gasp and it felt like I couldn't really speak. This made the guy, not sure at that point which of about 3 of them it was, even more bold I guess because he started to work his fingers down to my private area while the train went down this stretch for like almost 15 minutes. I don't understand why I felt too embarred to say anything during this whole thing. Why does this happen? I felt pretty foolish after it all.