There had been talk of a remittance tax over the last several years which would tax money exchange and transfers.
An income tax, in the form that many are know from their home countries is difficult and expensive to operate. However, a flat tax (of say 5% or 10%) is relatively easy to implement and manage if they simply place the onus on the employer to collect and remit it.
The big advantage to the income tax over remittance tax is that it is more even, collects more money and they can easily have a threshold where if you fall below it you pay no tax (or a smaller percentage) so that you don't hit the lower paid workers as hard (or at all).