As one of the first blockbuster superhero films of the 21st century, the first X-Men was a calculated risk, keeping things relatively small and not branching too deep into comic mythology. However, once X-Men proved the property's viability, X2: X-Men United was able to take the gloves off and unleash its claws.
Knowing what the filmmakers now had in star Hugh Jackman, X2 puts a lot of the focus on Wolverine's origin story, but it doesn't neglect any of the other X-Men either. In fact, X2 actually throws even more mutants into the mix, including fan favorites Nightcrawler and Colossus. Since X2, the X-Men franchise has changed drastically, and split off into three different franchises, but that huge boom can likely be traced back to this improved sequel.