Why is there a 3D option for Google Maps but not Google Earth?

From Street View in google Maps while viewing a panorama pressing the letter "t" or the number "3" enables 3D mode.

This feature, however, is not yet implemented in google Earth simply because google decided to first implement the stereoscopic 3D feature in google Maps presumably because there is a lot more users of google Maps than google Earth. With enough demand the google Earth developers may add this to a future version of google Earth.