Because in the previous movies, Letty was missing/lost her memories. Throughout Fast 5/6, Dom and Elena were seen together and getting really close. In Fast 7, however, Letty regained her memories and it helped rekindle the lost connection between Dom and her. As for Elena (assuming a time jump), she retired as a cop and working for Hobbs due to being pregnant but didn't reveal this until Fast 8. So that's why Dom really cared about Elena, for what they had and for conceiving his child. The one thing that Dom cares about the most is family. When she was killed, it triggered him emotionally.