How can so many laws about mitzvos and concepts that are vital to being a jew be forgotten? I understand that a few could be lost but it seems like almost every law was forgotten!

To the contrary, over 3,350 years since the Torah's giving, we are very proud that there are surprisingly relatively few (!) mitzvah details which have been forgotten! The reason is the continuous study and analysis of all of their many details in the Talmud, and the constant questioning and clarifying which is ingeniously "built in" to the system of Torah and yeshiva study. You are probably referring to the mitzvot which haven't been practiced since our being sent to exile about 2,000 years ago, e.g. some of the details of animal sacrifice and agricultural mitzvot, but even they were meticulously codified in the Rambam's Mishneh Torah. True, the Talmud Bavli, edited outside of Israel (about 1,500 years ago), sometimes ignored such issues (although it surprisingly often does!), and the Shulchan Aruch consciously passes them over, but the Talmud Yerushalmi, edited in Israel (about 1650 years ago) does deal with them extensively. Nevertheless, because the Yerushalmi is much more difficult (often leaving questions with no clear outcome), and there are much fewer helpful commentaries there, and in truth, these mitzvot were less necessary, accordingly it has relatively taken a backstage. On the other hand with the Torah world getting constantly stronger and the national return to the Land of Israel, there is a recent revival today in Israel to return to clarifying these unclear details with renewed study of the Yerushalmi and Mishneh Torah, returning to study Zeraim, Kodashim and Taharot, just like Moed, Nezikin and Nashim.