Just like the same reason Dwarfs always speak with a Scottish accent, Goblins have to speak in Cockney. It's just the law.
But seriously, it's not really as much an unwritten rule as it seems to be for Dwarfs, but it's a trope too. In large part it's because of Warhammer Orcs and Goblins, which usually speak / are written in Cockney.
Why not use different accents for goblins?
Well, they are one goblin society, it would make less sense that they spoke with different accents. Accents are usually socio-geographical in nature.
Beyond that, it's probably smart not to draw lines between a chosen accent and the culture the accent tends to be found in. Cockney is an dialect of the language English, much like Scottish and Irish is, so it's at least not some foreign accent they are applying.