Rocket League is an amazing competitive game that requires you to constantly learn both your opponents and your play in the game. It is so popular becuase of not only its competitive part, but its "freestyling" gameplay. There are many things in Rocket League that one can do to make their goal look flashy and make peoples jaws drop.
Another thing is the Market where people can trade with adding people in game via websites and buying and selling your items to get your dream Rocket League Car. (There is only 1 p2w Item; Sparkle boost because it doesnt show for 1/4 of a second).
Voicechat, chat filters and quickchats are great aswell as the toxic but creative community.