Because the Torah says that once a permanent place for sacrifices has been designated by G-d, sacrificing elsewhere is not allowed. The source for this is Deuteronomy 12:5-11
But you are to seek the place the LORD your God will choose from among all your tribes to put his Name there for his dwelling. To that place you must go; There bring your burnt offerings and sacrifices, your tithes and special gifts, what you have vowed to give and your freewill offerings, and the firstborn of your herds and flocks. (…) You are not to do as we do here today, everyone as he sees fit, since you have not yet reached the resting place and the inheritance the LORD your God is giving you. But you will cross the Jordan and settle in the land the LORD your God is giving you as an inheritance, and he will give you rest from all your enemies around you so that you will live in safety. Then to the place the LORD your God will choose as a dwelling for his Name—there you are to bring everything I command you: your burnt offerings and sacrifices, your tithes and special gifts, and all the choice possessions you have vowed to the LORD.