The host country has always alternated before and since that date. I cannot find any details but the ABC dispute with Kerry Packer/WSC might be a reason , can anyone confirm?
1977 was the centenary year of the Ashes. The first ever test match being held in 1877 at the Melbourne Cricket Ground.
By way of celebration, a "Centenary Test" was held at the MCG in 1977. This test took place before that year's Ashes series but did not count towards it.
Also, the Ashes series was moved to England during the 1977 Queen's Silver Jubilee year. This provided a further celebration of the centenary and Her Majesty attended the Lord's test.
My understanding is that the Kerry Packer controversy occurred too close to the start of the series to have had any effect upon the location.