youtube's strategy (e.g., 'the reason') was to leverage google's resources to undermine the revenue-making opportunities of the whole market so that it could outlast them to domination. it used google's (monopolistic) advantages in search and online advertising, as well as its capital warchest, to effect this strategy. that warchest, for instance, allowed it to offer free hosting/streaming, something that was difficult for smaller players to do. google's dominance in search allowed it to offer premium advertising to youtube videos. google's dominance in online advertising (again, monopolistically) gave youtube an advantage in monetization and targeting.
in short, this wasn't a story of a scrappy competitor overcoming the odds to become the market leader. it was hoarded capital and monopoly advantage being deployed to corner another market. that is, it was an (unprosecuted) anti-trust violation.
[0]: note the broader connotation of 'technical', not limited to just technology