Ultimately it's just market segmentation - people willing to pay a little more for portability.
However I think that it's worth bearing in mind that when the 2015 model launched it was the first of the 'next generation' of MacBooks with the Space Grey colour choice, tiered battery for a slim chassis, USB-C, top-notch speakers and... the butterfly keyboard.
It could command a bit of a premium in 2015 because the Air of that day was a non-retina dinosaur (albeit a very attractive and well-loved dinosaur). I feel that Apple eventually made the 12 inch MacBook pricing seem less premium by raising the price of everything else...