I assume because cryptocurrency pioneers are bitcoin, so people are more familiar with bitcoin than the altcoin. Then according to my own perspective and I experienced it, it's because the providers of these goods or services only accept bitcoin and most of them don't yet have altcoins. So the use of bitcoin to buy something more is seen. In my opinion, it is because fluctuations in the value of altcoin are faster and greater and cause great risks and worries for service providers and goods.