I remember that being brought up awhile ago and here was the answer (which seems ok to me):

On ETH, the Transaction Time/Fees are staggering...

On Hive, they're FREE and roughly 3 seconds... but Splinterlands has to pay for Server Rental for their game and all those Tx's which require processing... Which is why they have the fees and everything that will require a Hive Transaction will probably have a Tax/Fee associate with it to pay for the Server Costs...

Finally, this is why Splexx did so well at first we their Golems... but then why they upped the List Cost to 1 DEC... to reduce the reprice #'s and limit the strain of a bajillion Tx's per day...

That may have changed and may not be 100% accurate as it was from memory... but that's what I remember when that was brought up before