It says in the Maamer Basi Legani 5711 and all of the other Basi Legani Maamarim of the last Lubavitcher Rebbe, which were said (at least) one per year for a course of about 40 years, that the special point of the "Shittim" wood that was used to make the Mishkan, is because the word "Shittim" is related to the Hebrew word for "Shtus" -- simply translated as "folly", but it really means to turn, like leaning to one side or the other, like sitra achara -- the other "side", it literally means "side" or
turning to one side or the other; this could be expressed into evil, like turning to the side of evil "below reason", which is why its called "folly"; but the same word "Shtus" -- turning to a particular side, could also have meaning regarding "turning" to the holy side, turning to the side which is "beyond reason" -- holiness beyond simple understanding (as opposed to "folly" which is just below understanding), and the "Shittim" wood was used for the essential components of the Mishkan (as opposed to just the covering), because the entire point of the Mishkan -- the dwelling place for Hashem in this universe, is to transform the "Shtus" -- "turning" of the "evil side" (referred to as "folly") into the "Shtus" -- turning to the side of, holiness, and that is the entire purpose of the Mishkan, to transform darkness to light and evil into holiness, through the Avodah of "Hiskafia (subduing the evil inclination in one's day-to-day life) and Hishafcha (not only subduing the evil, but transforming it into holiness), and through doing that in our own divine service, that causes Hashem to dwell in the lower worlds, in you yourself, just like He is in the Mishkan.
So basically, the reason why "Shittim" wood was used for the Mishkan:
because the actual, physical Mishkan's entire idea was to be a dwelling for Hashem in this world, and the same is true for each of us to make a dwelling for Hashem, so it was made with Shittim wood in order to teach us a lesson of how to transform our own darkness into light, and also because that was the whole idea of what the actual Mishkan was based of, turning the "Shtus" of evil to the "Shtus" of holiness, see the above Maamarim at length...