Because the OSI is responsible for the creation of the term as a concept (Yes, I know you might be able to find one or two instances of some one saying "open source" in the context of software before the OSI came along, but the vast majority of usage and the ideas behind it came about because of the OSI).

More importantly than that- because the OSI definition is a darn good one and the BSL is a garbage license that restricts essential freedoms. The OSI definition ensures a standard set of minimum freedoms exist. Namely, the BSL makes it difficult for a fork to exist, brings additional restrictions that could need additional legal review, and prevents users from seeking alternative suppliers, locking them into an ecosystem that may not or may cease to meet their needs.