Although this is not specified why on bulbapedia. It is most likely due to the fact that it was one of the best Pokemon movies to release (performing overall second for two weeks in the box office, first in domestic movies for 3 in a row (then once more a week later), and in anime movies it performed first for three weeks and moved to second the following week but moved back up to first for the last 3 weeks of its screening.
The movies final figure was 4.3 yen (36.4 million US dollar) and placed SECOND in 2005 in japan only to be beaten by Howl's Moving Castle...
The most likely reason is that this movie generally did the best (although the rest usually did not have a chart like this) and performed very well so it would be hard to imagine why not. probably also for the fact it is back by popular demand.