Because to me, 'Broadcast' is a new and unique root. It is a root invented to describe a process that had no word to describe it, the sending of something, like sound, through the air on radio waves. The new root 'Broadcast' describes the process, and comes in two forms, a noun form and a verb form. As a new root, naturally it would take the standard -ed ending. The root is from the word 'to cast', as in to cast out something, but it is not a form of 'to cast'; it is merely a metaphor to create a new word. Nothing is being 'cast out' when I make a broadcast. You do not have to throw out, or cast out, the radio waves; they move on their own. It is like turning on a light- poof! and the radio waves, being a form of light, travel all on their own. Nothing is being 'cast out'. Hence to me it is a new and unique root, only using 'to cast broadly' as a handy metaphor.
And that is why I say 'broadcasted', where I very well know that 'to cast' becomes 'cast', not 'casted'. Because 'Broadcast' to me is a new root that only references 'to cast' as a metaphor and is not a form of 'to cast', because you are not 'casting out' the radio waves, but rather they move on their own as soon as you create them. Thus it is a new root that only refers to 'to cast' as a metaphor, and as a new root naturally takes the standard -ed ending.
Hence, I use 'Broadcasted'.