Why does another company not come in to steal 9% to 24% of the GDP of the whole country?
It's less than that.
$ 400 million commissions / $ 24 640 million GDP x 100 = 1.66% of GDP.
I am not familiar with how revenue is shared between the remitting companies, but part of those commissions are set to offset the costs of operating the remittance network in El Salvador. Operating offices, armored trucks, complaiace, bank staff wages and security expenses.
The situation is also getting better based on this report linked in Wikipedia:
Not in El Salvador. A quick Google search shows that:
A mid sized bank supports about 9 remittance companies. And a credit union supporting about 15.