Reply To: Building the Beis HaMikdash

There are a few reasons.
First of all, the Sifre (I believe quoted by the Ramban) says that a Navi is required to issue the command to build the Beis haMikdash. We don’t have that now, and won’t until Mashiach comes.
The Chinuch says that the majority of Jews must live in Eretz Yisrael, and it should be a sovereign Jewish nation. That is given as a reason why the 2nd Beis haMikdash was considered weak from the start – most people didn’t come from Bavel, and it wasn’t controlled by the Jews. The Gemara says the Shechinah wasn’t there for those reasons.
Until Mashiach comes and gathers the Jews together, we won’t have the prerequisites to build a new Beis haMikdash. Mashiach will be a king, and Eliyahu will be the Navi. We will have the gathering of the exiles to Eretz Yisrael. May it happen speedily in our days!