history - Why are most mitzvas impossible to keep?

Nowadays, one can only keep so many Mitzvos (271). That is less than half of the commandments of the Torah. I don't know if there is any statistic regarding how many Psukim discuss currently relevant laws in contrast to previously relevant laws (for example, how many Psukim discuss Tzaraas, Korbanos, and Tuma vs. Tzitzis, Tfillin, and Dinim).

Moreover, many laws became inapplicable towards the end of the first Beis Hamikdash. That means that there are many laws that were only applicable for less than 800 years.

Hashem knew that the Jews will spend most of our history in exile. Why is so much of the Torah written about things that will be actualized rarely, while many laws that are relevant for all time (for example, Tfillin, Mzuza) are learned from diyukim in scattered verses?

Answer

Maybe that's just it. With exile having been our dominant mode of existence for most of our history, there is a real danger that we'll come to see that as the norm. By having - and learning about - so many mitzvos, with the details richly given, that we can't perform in galus, then that drives home the point that things are not how they should be, which in turn should spur us to become worthy of geulah.

Another possibility, too, is that the details of those rarely applicable halachos are more likely to be forgotten altogether. (We even find that the paradigmatic forgotten halachos, those that were lost during the period of mourning for Moshe, include cases involving kodashim - Temurah 15b-16b.) At least with tefillin and tzitzis and mezuzah, since they are daily activities, their basic halachos are pretty clear and unequivocal; the machlokos about them tend to involve the details. (Everyone agrees, for example, about the shape, color, and positions on the body of tefillin, and on what parshiyos are in them.) By contrast, if the details of korbanos, tzaraas, and similar mitzvos weren't given in the Written Torah, it is quite possible that substantial chunks of this information would have been lost or confused (the more so because there are a lot of subtle differences between ostensibly similar cases).