Eye and cursor tracking research repeatedly showed that the user's view results in an F-shaped pattern. This was first discovered by Jakob Nielsen. Later on, experiments on google showed a triangle shape, which is related to the F pattern, as the most common pattern, as in this image below:
Source: How Do You Google? New Eye-Tracking Study Reveals Huge Changes
Keeping this basic format means making the user's search easier and more intuitive, as the access to relevant content is easier, and the content is thus easier to process for users. These are topics of UX design and usability. It's about the users' comfort.
If you look into the source link of the picture, you'll see that there are indications that this pattern may have changed to a straight downward line at the left side of the stream. Where changes come from, I don't know, probably an effect of mobile communication. However, also if this new pattern holds, the left part of the screen still receives the most attention. It's thus logical, from a UX viewpoint, to place the most relevant content on the left.
I do wonder if this effect is also due to the left -> right orientation of the Latin alphabet.