Firstly, neither Polygon or Wax existed at the time.

And then, much of the value used to create Splinterlands in the first place was derived directly from Hive (Steem at the time) through various means, including bidbots. And, the vast majority of early adopters went into Splinterlands because it was on the blockchain they use and have value, as well as the social capital built with Aggy and Matt over the years that garnered support - for example, I spoke to them both and had drinks with Aggy in Poland at SF3, as did many others. On other blockchains, they would have been starting from scratch and had none of the social trust built, nor personal relationships to help them get started.

People seem to forget this when they look back into the past using the filters of today.

Not sure why you think it is hard to trade, at least Binance has it paired with BT/ BUSD/ USDT there and Bittrex has pairings.