Because the last Time Lord died and needed to be replaced, which is a recurring theme in the Doctor Who series. They regenerate into a new person, even though they have the same memories and some of the same quirks as previous incarnations.
Matt Smith won the role by wowing the head writer and casting director with his unique portrayal of the Doctor.
Matt smith (Doctor Who)
Matt Smith is the new Doctor.
No, David Tennant plays the Doctor in that episode. Matt Smith isn't in it.
Yes, he is.
Yes.
Yes, Matt Smith is currently portraying the Doctor in Doctor Who.
The new doctor is Matt Smith.
Matt smith (Doctor Who)
Matt Smith is the new Doctor.
Matt Smith
Matt Smith
No, David Tennant plays the Doctor in that episode. Matt Smith isn't in it.
Matt Smith is the eleventh Doctor.
Matt Smith.
Matt Smith.
Matt Smith is only 26 years old
The next Doctor in Doctor Who shall be Matt Smith.