...Is it because there are not many people trading ‘PI’ on Bitget and therefore, if I’ve have been trading one of the bigger coins like BTC or ETH, it wouldn’t have happened? Or is there an entirely different reason?
In your case, the problem is that there wasn't enough liquidity to execute your stop loss. Thus, your order was closed gradually, as counter orders placed by other traders became available. You can avoid this if you set a stop-market rather than a stop-limit.