this assigns uart[channel]:
always @(*) begin: uartmux
        integer i;

        for (i = 0; i < NUART; i++) begin
                if (i == channel)
                        uart[i] = uart_tx;
                else
                        uart[i] = uart_tx;
        end
end

this doesn't:
always @(*) begin: uartmux
        integer i;

        for (i = 0; i < NUART; i++) begin
                if (i == channel) begin
                        uart[i] = uart_tx;
                end else begin
                        uart[i] = uart_tx;
                end
        end
end