First entries in a video game series have a lot of hurdles to overcome. They need to introduce the characters, story, world, and lore, plus all the different gameplay mechanics. They also often try to do something more experimental than other series on the market to stand out. For all those reasons, plus many more, it usually isn’t until a sequel that most games really begin to shine. This is when the developers really know what their game is from the start and can build off of what came before, knowing the player also has that knowledge. Of course, that also means expectations are much higher, leaving many sequels that don’t do quite enough to feel disappointing.
source: Digital Trends