It's a mix of everything from the clogged up bitcoin network desperately in need of being able to handle more transactions to additional fees people tag onto bitcoin. Then there's those that don't allow an option for transaction priority or just leave it stuck at some obscene value when bitcoin was at $4000.

You also have to remember that not many blockchain networks are even remotely ready for large numbers of transactions and going mainstream. Ethereum network is getting hammered just from cryptokitties, over 700K and it's in trouble. On the other hand Steem barely breaks 0.25% at 700K!!