Comment by egypturnash

This distinctive movement is a compromise with the limitations of a 2mhz 8080 with no video support hardware to speak of.

Looking at a commented disassembly (https://www.computerarcheology.com/Arcade/SpaceInvaders/), there's so little time available that each frame redraws nothing more than the player, their shot, one of the 55 aliens, and one of the three alien shots. Four things. When every moving object has to be manually erased and redrawn within the space of one frame, you kinda have to stagger the movement.