Note the mobile skin is separate from the mobile site. If you set in your preferences the skin Minerva Neue, you will get it on desktop (just the skin, there are non skin differences with the mobile site you wont get).
As for why not default- i imagine partially because the mobile site tends to be unpopular with power users (of course opinions vary)
As for why not default- i imagine partially because the mobile site tends to be unpopular with power users (of course opinions vary)