Why did they call the space "vacuum"
They called the space vacuum because it is an area of space that has lower air pressure compared to its surrounds. Space has no air, which makes it to be the greatest vacuum of all.
Because of the lack of pressure. There is no pressure in spaceAt sea level pressure is 14.696 psi
A volume is at a vacuum with respect to some other volume if it has a lower pressure. Space is at a lower pressure than earth's atmosphere so we say it is at a vacuum.