This really is one of those situations where “it’s not a bug, it’s a feature” holds true.
The original Space Invaders was written to have the aliens maintain a constant pace throughout game play. But the machines took time to render the aliens. As they were killed and removed from the screen, less rendering was required, which cause the play loop to complete faster and the aliens to speed up.
You are probably wondering why the entire game didn’t speed up then, and in fact, it did. In future version, this portion was fixed, but the increasing speed of the aliens was left in.