Because before Tesla launched the feature, "Autopilot" was known in the general consciousness to mean a specific thing. Autopilot is the thing on planes that lets the pilot do other tasks (like navigational routing).

Other car manufactures are smart enough to not brand their cruise control as "Self Driving" or the like.

If I sold a brand of drain cleaner under the name "Laundry Detergent" that would rightly result in some consumer protection investigations.