"Why did Blackmagic do this?
It's hard to say. The most logical answer is that there are hundreds of lenses in EF mount, which is an argument for the decision, but they are (for the most part) still photo lenses. They're an affordable way to get your hands on lenses with the optical quality to match the camera, but Blackmagic could easily have gone RF or possibly L-mount then provided a $100 adapter to mount EF lenses on the camera body. "
That doesn't look like knowledge or reason. The only idea expressed is they wanted to move on from the EF.
Only other mount that would make sense is a MFT or offer interchangeable like on the Ursa's. RF is not open for licensing yet and was only agreed with RED due to an IPA w/Canon. Leica has it's L Mount alliance with Panny, Sigma. BMD is not part of that alliance so that kills the L and RF right off the bat. A little research or knowledge goes a long way before a clickbait title.