Why we dont have Yibum ( leverant marriage ) but we have Chalitsa these days ?

It's already many generations that we don't do yibum because the intentions of most people wouldn't be to do the mitzva but rather improper thoughts, so the chalitza is what's done. In any event, even chalitza is rather rare today, for Baruch Hashem, technology can solve more than 90% of the fertility problems in couples, not to mention the vast majority who don't have problems conceiving, and the non-religious couples who usually don't get married until they want children.