Because the title is relevant to the episode, and to parters simply mean different episodes that are linked.
RTD did give a single name to one two-parter, "The End of Time". The only explanation he's given for doing that one as a single name, and others as two names, is that "It seemed appropriate to call the whole thing "The End of Time", just like it seemed appropriate to have different names for Stolen Earth and "Journey's End"".
Trying to dig deeper and figure out _why_ it seemed appropriate, all we can really do is guess. But the new series does try to make each episode stand on its own, equal to all the others, and giving them all names helps do that. Especially if the names are relevant to the episode.
And it's not as if he was breaking some sacrosanct tradition. When the show started in 1963, they had names for each episode. They later added names for each complete story, and then dropped the episode names (and then there was "The Trial of a Time Lord", which only had a name for the entire 14-episode season). So, you could just call RTD's usual naming scheme a return to the original 1963 system.