Because the astronaut shot him whilst he was regenerating. A Time Lord can be killed out right if they are between regenerations. This shows us that even Time Lords are not invincible.
An Astronaut
The Christmas Carol episode of Doctor Who was aired in 2010 with Matt Smith as the 11th doctor. The following episode of the series was the Impossible Astronaut.
Eventually yes, he will regenerate into the twelfth Doctor.
No. A timelord can regenerate 12 times, Matt Smith is only the 11th doctor. Also the master got around the whole 12 regenerations thing so I would hope that the doctor will too.
The 10th Doctor, played by David Tennant, regenerates by absorbing a large amount of radiation while saving Wilfred Mott in the "Doctor Who" episode "The End of Time." This process causes him to regenerate into the 11th Doctor, portrayed by Matt Smith.
The 11th Doctor in Doctor Who is portrayed by actor Matt Smith.
Matt Smith
He hasn't yet. The tenth doctor will be regenerating into the 11th Doctor around Christmas/New Year of 2009.
Only certain people know about what happens in the series, but the chances are that he won't as Matt Smith is still quite a young actor and shouldn't have another job offered for him yet. I think that it's fair to say that the 11th doctor will be staying for at least a bit longer.
They will easily find a way around it like they have done with the Master getting a new cycle of regenerations. As the characters are always saying time can be rewritten.
The Eleventh Doctor, portrayed by Matt Smith, is a fictional character from the television show "Doctor Who." He is an alien Time Lord who has the ability to regenerate when he is mortally wounded, so he does not have a specific age like a human being.
11th doctor