I believe this is because many languages will interpret that differently, and Wikipedia having contributors from around the world writing in different languages, decided that clockwise is a good universal instruction that is more difficult to mistranslate.

If the instructions were "left to right" and then "top to bottom", a bad translation might end up making it mean the top to bottom first and left go right last.

Reading order

In Arabic (and a few other languages too), things are read RTL, and plenty of Asian languages, at least traditionally, are read top to bottom first.

Doing it clockwise is seen as a more universal instruction I guess? And I don't think it's too hard to figure out. I might also be to make the instruction more concise!

Clockwise order isn't the same as reading order. The correct order is:

1. The top left image (because it says "from top left")
2. The top right image
3. The bottom right image
4. The bottom left corner

It helps if you imagine a circle around the images with arrows pointing in clockwise direction, like this:

(EDIT: I made a correction to the drawing. I put the wrong order in the original drawing.)

They could have just put the images in reading order though, especially when there are only 4 of them. It would be way less confusing :/