Because it was never finished. Filming of "Shada" was halted due to a technicians' strike, and by the time the strike was over, the Doctor Who production team had run out of their allotted time for studio shooting for that serial. So, they had no choice but to scrap it.
Shada.
Doctor Who Shada - 2003 is rated/received certificates of: UK:U
Doctor Who Shada - 2003 1-1 is rated/received certificates of: UK:U
Doctor Who Shada - 2003 1-3 is rated/received certificates of: UK:U
Doctor Who Shada - 2003 1-4 is rated/received certificates of: UK:U
Doctor Who Shada - 2003 1-5 is rated/received certificates of: UK:U
Doctor Who Shada - 2003 1-6 is rated/received certificates of: UK:U
Doctor Who Shada - 1992 V is rated/received certificates of: UK:PG (DVD rating) UK:U (VHS rating)
Stuart Crossman has: Played Mr. Bates in "EastEnders" in 1985. Played Stu in "Weekend Bird" in 1997. Played Motorist in "Doctor Who: Shada" in 2003. Played Krargs in "Doctor Who: Shada" in 2003. Played Constable in "Doctor Who: Shada" in 2003. Played Dalek Operator in "Doctor Who" in 2005.
Originally, Zack Shada voiced him but once his voice changed, Zack's brother, Jeremy Shada took over the role.
The Antique 'Doctor Who' Show - 1993 TV is rated/received certificates of: UK:U UK:U (DVD release)
Expanding your question to 2008 (as it is now as I'm answering, so up until the last episode of Season 4 of the new series, Journey's End)... There have been 202 stories (if you include the unfinished Shada) comprising 757 episodes. And if you don't count Shada, 201 stories 751 episodes.