Maybe this because of personal historical perspective but I think I have an idea as to why. PHP7 wasn't released yet and FB was the creator of it. I think those

Maybe this because of personal historical perspective but I think I have an idea as to why. PHP7 wasn't released yet and FB was the creator of it. I think those two decisions influenced using Hack (as the staff may have been big PHP users) instead of waiting for PHP7 and how it would be adopted/grow. Also, it supports typing arguments which was probably the idea to handle complexity.