MR_SLINThe reason is that it's hard to consistently get teams into the casting mumble before matches, you can't control what they say, and it didn't seem to add much to the cast. A lot of professional esports don't do this for the same reason, and if they do include it (like in LoL, for example), it's edited in post.
They still do after game interviews though, you can't really control what the players say in those either no?
As for the professional esports comparison, fair enough really. As I mentioned before the reason why I thought it was so cool was because you don't see it done in other games so it made TF2 a more entertaining experience to watch for me.