Goblins and Merfolk are two of the creature types that have been around since the very first Magic Set was released almost 25 years ago. Although they both had a Lord already with Goblin King and Lord of Atlantis, both tribes were initially represented on very few cards. But some sets later, the first synergistic tribal decks started showing up, in a similar form as we have them for various other creature types nowadays. Although individual goblins are generally small and weak, their creative approaches to solving problems often lead to very interesting activated abilities, some of which later turned out to be quite overpowered. From Block Constructed to Standard, Extended and Legacy, the little green dudes on red cards repeatedly made it to the top of various constructed environments, and with Goblin Recruiter one of them even made it onto the Legacy Banned list.
Merfolk decks became popular a bit later, but have been successful in Modern and sometimes even in the Eternal formats for years now. Sounds like a good reason to let these two funny little tribes compete against each other directly on the basis of these new duel decks!