Why does the angel teach the fetus the entire Torah in its mother’s womb?

Hello, The source for this is in the Gemara in tractate Niddah (32b). The explanation is as written in the book Maalot HaTorah (Jerusalem 1989, pages 226-227), in the name of his brother, the Gaon Rabbi Avraham of Vilna, may his memory be for a blessing, regarding the words of the Gemara in Niddah cited above: apparently, one must understand what benefit there is in teaching him and then causing him to forget, since it is as though he had not learned at all. It seems, according to what the Alshich wrote regarding what we say, And give us our portion in Your Torah, because all souls stood at Mount Sinai and each received his portion in the Torah. This is what is written in the Gemara (Megillah 6b): I labored and found, believe it; found specifically, like a person who finds his lost object, because it is his portion. According to this, if he had not been taught while in his mother’s womb, how would he be able to attain it through his toil? And if he had not forgotten it, he would not need to toil. If so, reward and punishment would be nullified. Therefore, they teach him his portion of Torah in his mother’s womb, and this is called the entire Torah, meaning that which pertains to the portion of his soul. And when he comes out into the air of the world, it is forgotten from him, and afterward, through his toil, he can find his lost object. So wrote the Etz Yosef on Niddah there. 

With blessings, Hillel Meirs