But I must joke a bit: did you ask the same question to the I2P devs when they forked Freenet? :P
Seriously though, they are different projects with different goals and vastly different functionality. The only overlap is "privacy".
There simply isn't anything like Web420, and I think this is a better approach to getting the average Internet user away from the big tech/VC-backed web apps that are ruining the world, that's why.