The Next Doctor aired on 25 December 2008, Planet of the Dead aired on 11th April 2009, The Water of Mars aired on 15th November 2009, The End of Time Part I aired on 25th December 2009 and The End of Time Part II aired in 1st January 2010.
There will be no Doctor Who series in 2009, instead there will be four specials, the first of which will air on BBC1 on Easter Sunday. There will however be a new series in 2010.
There will not be a new series of Doctor Who until 2010 when the new Doctor will have taken over from David Tennant. Planet of the Dead is the first of four specials that will air in 2009 instead of a full series.
Not much has been confirmed yet. We know that will be four specials in 2009 but we do not know what the titles will be or when they will air. Filming for them will start in January. Two of them will be written by Russell T. Davies, one will be written by Davies and Gareth Roberts, and the other will be written by Davies and Phil Ford.
If you mean 'why does the next full series start in 2010', then that is because in 2009 they are having four specials instead of a full series because of the BBC budget and the change over of control on the show, they have decided to have specials, and return with a new series with Steven Moffet at the helm in 2010.
There is not a new series of Doctor Who being made until 2010. During 2009 there are instead four specials, which will air throughout the year. Exact dates vary depending on the channel and country.
Connect Four - 2009 was released on: USA: 2009
Four Dimensions - 2009 was released on: USA: 30 September 2009
There will be a Christmas special this year on 25th December, but the next series will be Spring 2011, and will be known as Series Six.
Four Years - 2009 was released on: USA: 23 April 2009 (On Location: Memphis)
Two Four Six - 2009 was released on: USA: 15 January 2009
Four on the Floor - 2009 was released on: USA: 1 January 2009 (DVD premiere)
One Nine Nine Four - 2009 was released on: USA: 9 October 2009