Because Christopher Eccleston had a very prominent Northern accent, and they wanted to to just sweep that under the rug right away. Also it does somewhat address the weird thing that an alien has a British Northern accent. Also it's pretty funny.
That is also one of the reasons David Tennant used an English accent instead of his natural Scottish one. They didn't want to make that a thing, with having to explain accents to new viewers. Since the series had already cemented itself by the time Capaldi was cast, they knew no one would care if the Doctor now had a Scottish accent.
While it is perfectly acceptable for aliens to have a accent from the South-East of England, one from any other part of England is somehow seen as weird. As if it wasn't weird that they should have an English accent at all. The programme makers obviously felt that the fact that the character didn't have a South-East England accent was an issue that had to be addressed.