Why is my sim locked and how can I unlock it without contacting my carrier

If just your SIM is locked the only way you can unlock it is by talking to your cell service provider.

If you are referring to a carrier lock and you are trying to use a SIM from a different company you will have to contact your carrier and ask them what their unlock policy is and if you qualify.

SIMs belong to and are controlled by the carrier, as well as the phone itself depending on circumstances. That is why a carrier can "blacklist" to refuse any service anywhere if it chooses.