Because there's more to a computer than raw CPU benchmarks. It's the thinnest MacBook product, the lightest, has a Retina display, and an extremely fast SSD storage.
That's what makes it a 2015 notebook. It is over priced, but not dramatically.
Why do people insist on ignoring the fact that it is bloody fanless?
Not only does it make it quiet and pleasant to use, it makes the laptop extremely reliable, and the performance won't degrade over time due to bad airflow and throttling. It's a huge thing for laptops.