The Mishna Brura mentions that people used to blow the Shofar at NIGHT during Chodesh Elul where did they do this ? and why haven’t we continued this today?

The Mishna Berura (581 3) writes that the Shofar is blown during Elul in the mornings. It is the R'ma (Orach Chayyim 581 1) who writes that there were places where the Shofar was blown in the evenings. Rav Moshe Feinstein (Shu"t Iggrot Moshe IV 21) explains that the custom in these places (referred to also in the Tur and Chayyei Adam) was to blow before sundown. Every place has its custom, and the generally accepted one is what prevails today.