For the same reason StackExchange breaks up their Q/A sites into niches. It's a better experience for the user. It creates a community around the niche. A brand. It builds momentum faster. What if StackOverflow, MathOverflow and Startups were all part of the same general site? It would get messy. It would lose a lot of its value from being domain specific.

Also it's better for advertisers.

It's a good idea. There's nothing wrong with what Mahalo is doing here.