Why was the cackling goose split off as a distinct species from the Canada goose in 2004?
The "white-cheeked" geese have long been subject to taxonomic controversy. From the 1920s to the 1950s, authorities have classified them into between one and four species. Although Aldrich in 1946 asserted there was near-unanimous agreement among Arctic biologists that there were two species, from the 1950s Delacour's taxonomy was most often followed, which had one species (Branta canadensis) and twelve subspecies.(1)

What changed the consensus were DNA studies.  Mitochondrial DNA (mtDNA), which is passed on without recombination, showed distinct sequences supporting two species. One RFLP analysis of mtDNA suggested the split between these two species happened about one million years ago.(1) 

The species divide mainly along size and breeding location lines.  The Cackling Goose (Branta hutchinsii) is smaller and breeds on the Arctic tundra. The (present) Canada Goose species (Branta canadensis) mostly takes a larger form and breeds mostly in non-tundra areas.(1)   

(1) - Mlodinow, S. G., et al. "Distribution and identification of Cackling Goose (Branta hutchinsii) subspecies." North American Birds 62.3 (2008): 344-360.