Because it was set up from the beginning that the film would lead into Godzilla vs Kong More on reddit.com

No, they're not connected to King Kong (2005) at all. Kong: Skull Island started as a planned prequel to King Kong (2005) at Universal set around World War I (and, interestingly, was set to be directed by Adam Wingard), but then the studio decided to hand the project off to Legendary Pictures (letting Legendary produce and they would distribute) and Legendary decided to retool it into a standalone movie where Kong is first discovered during an expedition in the 1970s at the tail end of the Vietnam War. This was around the time Legendary was filming and in post production on Godzilla (2014), and after the movie opened to such success and the greenlight was given to develop two sequels (what became Godzilla: King of the Monsters and Godzilla vs. Kong), the idea was presented to add references to Godzilla into Kong: Skull Island which developed further into having Bill Randa, Houston Brooks, and San Lin actually working for Monarch since that organization was Legendary's creation and they could add them into whatever project they wanted. Naturally, that lead to the executives going "If we're going to reference Godzilla and we're already planning to do two more Godzilla movies, since Cinematic Universes are so hot right now might we as well combine these into one?" Since Kong is a public domain character Legendary was free to make a Kong movie of their own, they just needed Universal to agree to give up distribution rights so Warner Brothers could co-produce - they traded Legendary's rights to some Universal projects (I forget which - I know Universal was pissed about it later, because K:SI did well and the movies Legendary traded it for were bombs and disappointments) in exchange for taking K:SI and then selling 25% to Warner Brothers so they would distribute. More on reddit.com

I liked this movie a lot. Sam Jackson's character seemed authentic in that he believes in an eye for an eye. While most of the other characters were in awe of such a majestic beast, Sam's character was like "you killed my men, I owe you the same". The lack of Kong wasn't nearly as painful as Godzilla 2014, and the monster fights in this movie were waaaay better than Godzilla. The soundtrack was good and John C. Riley was as good as he's ever been in that character. Not an award winner, but for what it is and tried to be I give it an A grade. More on reddit.com

Kong's introduction is pretty mighty More on reddit.com