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!