The Tolna Rebbe had a fascinating (and humurous) explanation of why the 3rd Beis Hamikdash will come down from Heaven.
Picture for yourselves how this Bais Hamikdash will be built. Let me tell you what is going to happen.
“The first ones who will enter the picture will be Agudas Yisroel who will say ‘Of course we are the ones who will build the Bais Hamikdash. We are the ones who worry about all the needs of the chareidi community. Is there any doubt that we should be the ones in charge?’ They will convene a whole assembly of gedolim in order to decide what needs to be done.
“Meanwhile, R’ Meir Porush will come and say, ‘And what about me? The Agudah represents everyone? I want this to be under my auspices.’ And then there will be a second group of Rabbonim discussing how to build the Bais Hamikdash.
“The Litvaks will come next and they will start to scream. ‘Gevald! The chasidism should build the Bais Hamikdash? They will all be dancing because they don’t know anything about the actual halachos of building the Bais Hamikdash. They will offer to find one Kohein who actually knows the halachos of the avoda. But they will tell us: “for this part, you are going to need the Kollel Chazon Ish. For that part you are going to need the avreichim in Brisk.” What are you, crazy? We will build the Bais Hamikdash.’
“Then will come Satmar, from both sides, and they will say, ‘You are all Zionists! You are unfit to build the Bais Hamikdash.’
“The Chabadniks will also get involved. ‘You are all crazy! Moshiach is ours! It is our Rebbe, our Moshiach, what are you talking about the Bais Hamikdash? You are all insane.’
This is so true, it is hard to believe that a Chasidic Rebbe would say this.
Source: Jerusalem - Tolna Rebbe: Why We Won't Be Able To Build The Third Bais Hamikdash On Our Own