Because they are 4 equals supporting and embracing eachother which led to a very consistent sound. LM also experiments with sounds, but the basis is still the steady and that delivered an other solid effort with Glory Days. And it shows in their sales. Not only their succes in the UK, but they are also slowly conquering Europe and Australia etc.
5H beside all the drama also served an amazing album imo. but their album is misleading for the overall sound they present to the GP. All chosen singles basically promise a different kind of album. The album is mainly filled with midtempo pop songs, while the singles, or atleast the first two take a urban/pop route.