Took emergency contraception (2hrs after sex), 19 days (2days missed period) later had negative quanti. Hcg blood test. Period still doesn't come. Why?

Many reasons. While it is unlikely you are pregnant, that may still be an issue, so pay attention to how you feel as emergency contraception will not work with an ectopic pregnancy the same way. Many things can alter periods, including stress, exercise, medications, and even age. If your period has usual been very regular, this last cause is unlikely. Also, in the future please use regular contraception.