I think a lot of it is the fact that dota is a complex, massively replayable game that valve made easy to get in to with detailed tutorials, the account leveling system, and making it free to play. If CS:S was free to play I don't see it rising as much because it doesn't separate the new players from the experienced ones, and the tutorial helps with basic concepts on how a match works, but not much else.
I think a lot of it is the fact that dota is a complex, massively replayable game that valve made easy to get in to with detailed tutorials, the account leveling system, and making it free to play. If CS:S was free to play I don't see it rising as much because it doesn't separate the new players from the experienced ones, and the tutorial helps with basic concepts on how a match works, but not much else.