Good question. While I cannot answer exactly why, I have some observations. It would definitely add cost. Using a chroma key in this way is a compromise since you cannot use the key color in your graphic. What most people do for this type of key is to provide a key and fill signal separately which allows any image in the fill and will work on all downstream or upstream keyers without using more complex chroma keying.
Regards, Gary
Gary Adams
Sr Product Specialist
Blackmagic Design