It is quite obvious if you look at the German language origin of the word.
To illustrate his concept, Minkowski (not Einstein) had the choice between "Raumzeit" (spacetime) and "Zeitraum" (timespace). However, "Zeitraum" was already an existing word in German, meaning "period of time", as in e.g. "Der Zeitraum zwischen Sonnenuntergang und Sonnenaufgang heisst Nacht." (= The period of time between sunset and sunrise is called night.)
Basically timespace was taken, so he settled for spacetime.