In 1514 The Flemish artist Albrecht Durer did two engavings:-

The first of a bagpiper showed the piper wearing a belted tunic over 'leggings' and with a decorative pouch worn in today's conventional sporran position.

The other of two peasants dancing shows the women wearing a pouch similar to the modern side sporran low on her right hip.

All suggestive that in the pre-pocket era it was normal to wear some sort of pouch: albeit where convenient to the wearer.

Later it was normal in the 18thC-ce for soldiers to wear their ammunition pouch front and centre of their belt: allowing easy access to their ammunition. However clear of their other military impedimenta such as a sword/bayonet etc.

This all suggests that the conventonal placing of the sporran is due to two factors:-

First the heavily armed highlander-for they were renowned for being over armed needed to wear it in a position where it would not get mixed up with/impede use of such things as claybeg, dirk, dags.

Second that the Victorians with their love of uniformity and military precision decreed that it should be worn in the present position.

As an aside, it is interesting to note how often in military photographs circa the Boer War and WW1, the sporran was not worn.

James