Why do nowadays people get "engaged" and then do the halachic Kiddushin and Nisuin at the wedding and they don’t do it like it used to be done, the halachic Kiddushin which is the "engagement" and then the Nisuin at the wedding?

After Kiddushin the bride is in many ways "married" and if one of the parties has a change of mind, a "Get" - a formal religious divorce - is required. The situation creates serious problems that are avoided by the accepted custom.