It can't be answered with any certainty as it likely had a cultural reason, the same way the earlier figure of 8 shields used by the Mycenaeans might seem an odd choice.
Smaller flatter round shields do appear in other parts of Europe during the bronze age and it could be something as simple as you can trace the outline of a shield with a simple compass made of a scribe attached to a piece of string. The bronze age saw the dominance of geometric art which was squares, lozenges, zigzags and also concentric circles. How that influenced the famous Greek shield we can only speculate.
Elongated or oval shields did become very common during the iron age and the Greeks themselves adopted a type they called the thyreos which they possibly copied from the Romans.
Something else to consider is that a common method of fighting was the spear held over the head and used in a downward stabbing motion, the legs wouldn't be in much danger in mass combat. It also needs to be considered that finances played a role in ancient battles as much as today and attempts to keep costs down were undertaken so some societies only had a greave on the left leg as that was the leg most likely to be forward.
Whatever the reason, the shield came full circle by the end of the western Roman Empire and round shields became the norm again. They persisted during the middle ages as the buckler and the last use of a shield in battle by westerners was the Scottish Highlander targe which was a small round shield.