Why do faculty use the software "Blackboard"?
Blackboard is used by the university I teach at and I used it at the university I studied at - 

Blackboard (BB) is mostly uniform. While you can customize your courses, to an extent, a student moving from one course to another using BB will already know where everything is. BB is simplistic and students have different technical abilities. 

Having each instructor learn HTML is a large order. I teach with an 80-year old man who still has a flip phone and can barely use that. Contracting someone to create the webpages would also be an issue because costs and instructors changing the courses that they teach may want to change the webpage. 

BB also has (at least where I studied and teach) a copy checker. If a student from Z class uploads the same paper to both Z and Y then when they upload it to Y the instructor is told that there is a paper within the servers (goes back 2-3 years I believe) that is x% similar to a paper submitted in course Z.