Almost a month has passed since we started reading about the construction details of the Mishkan. I'm sure that more that a few people are sick of the topic. Why is the Mishkan described in such excruciating detail? There are many other complicated halachic topics which receive only a line or two in the written Torah, with everything else being part of the oral tradition. Whereas with the Mishkan, all the technical details are included in the written Torah. Why is this? A friend of mine asked me this, and it bothered me until I came up with not one, but two possible answers.
1) The "practical" answer. One reason we need an "oral Torah" in addition to the written Torah is that written texts are inherently insufficient as halachic sources. They fail to cover every situation that might arise, and only with difficulty can they provide guidance in situations which do not call for a "yes or no" answer. Therefore, at least some halachic details must be transmitted orally. In practice, many mitzvot - for example lulav, divorce, and shechita - have just a single "placeholder" verse in the written Torah, along with an extensive body of oral law which does the real explaining.
But this explanation for the oral law does not apply to mitzvot such as the Mishkan. The Mishkan was built once, in the past, and will never be built again. Uncertainty as to how to perform the command is not a problem when there will never be another performance. Furthermore, there is a historical consequence to this lack of relevance. It is doubtful whether the laws with no practical consequence would be extensively studied; most likely they would be forgotten and disappear entirely (this has already happened to much more relevant laws). For these reasons, there is no advantage, and in fact a disadvantage, to the laws of the Mishkan being included in the oral Torah.
But the laws of the Mishkan are still laws. If they are not in the oral Torah, they must be recorded somewhere else. And there is only one option for the "something else". Thus, these laws must appear in all their gory detail in the written Torah.
2) The "hashkafic" answer. Many, perhaps most, of the mitzvot we do are not mechanical observances, but rather call for initiative and creativity on the part of the mitzvah-doer. If you want to give tzedakah, for example, there are many possible options. You could give cash to the guy on the street corner, or you could volunteer at a soup kitchen, or you could start a job-training program to help poor people support themselves. There is no verse in the Torah that tells you which of these to do. There is only a general goal, which is up to you to meet in the best way you can think of. Judaism encourages you to invest yourself not only in the performance, but also in the understanding and planning of mitzvot. This is an all-encompassing challenge and opportunity limited only by the final destination, "tikkun olam bemalchut Shaddai", which we have not yet succeeded in reaching. Anyway, the oral law, which is transmitted and applied only by virtue of human creativity, is the appropriate means of transmission for these mitzvot. (See "Halachic Man" for a fuller and clearer explanation.)
But there is one kind of mitzvah which works differently. The Mishkan represents the encounter between God and humanity. And when God and humans meet, humans must be aware of the asymmetry of the encounter. Basically, God is everything, and we are nothing. All of human creativity is meaningless when compared to the fact of God's existence. The Mishkan is therefore the site not of human initiative, but of human awe. There can be no freedom of choice, no personalization, and no oral Torah when it comes to the Mishkan. Unlike with other mitzvot, every tiny detail of the Mishkan is dictated by God, and therefore must be recorded as such in the Torah's written text.