Why does the priest say the words of the consecration in a low tone in the EF when Jesus probably did not say it in a low tone?

In the Extraordinary Form of the Mass (the so-called "Tridentine Mass" or "Latin Mass") the words of the consecration are said in a low tone. In the Ordinary Form they are said in what I think is called vulgar tone. Why does the Priest say the words of the consecration in a low tone at the Extraordinary Form when Jesus probably did not say it in a low tone?