From my perspective it was ease of use. I remember back in the day ( ~2009 ) that youtube became the defacto standard because its ui was simple, it loaded quickly, it was straight to the point, free, didnt require login and you could upload content straight from your android phone after recording it. So sharing was pretty much instant.
After that they came up with policies to help people create actual content ( reward content that had thought put into it, reward creators etc ) which helped with the rise of quality content and make youtube in the youtube it was today.