The owner of the work sells the rights to display their copyrighted material. Lulu pays for the rights to distribute the content within the US. It would have to pay more for the international distribution rights, plus the rights would have to be individually negotiated, but it chooses not to because its advertisers are not willing to pay more for coverage in areas where they have no sales or presence. Also, in some cases, a local distributor has bought the rights to display the work.
In the case of Hulu, they have chosen to not buy the rights for anywhere outside the US.