A brief explanation why…
Developed by a consortium that is now called the Bluetooth Special Interest Group, the technology saw initial contributions from companies including Ericsson, IBM, Toshiba, Nokia and Intel, the last of which employed Jim Kardach who proposed using "Bluetooth" as a codename until a more formal name was established.
The reference goes back to Harald Bluetooth, king of Denmark from c. 958 - c. 986 and king of Norway from c. 970 - c. 975/986. It's said that Harald united Danish tribes under a single kingdom and Kardach hoped that Bluetooth would likewise unify communication protocols.
Origin stories for Harald Bluetooth's name vary depending on who you ask. It the most commonly cited explanation is that he loved blueberries but it seems there's no historical source to confirm this. A more plausible explanation is the possibility that he had a "blue" tooth, which would have actually meant it was dark or black in Old Norse, likely referring to a bad tooth.