Although there are many similarities between them it is important to remember that conventional sports has had a much longer time to develop than e-sports and currently can't be compared popularity wise. Ever since starcraft (brood wars) it is however noticeable that e-sports is rapidly growing, and huge pricepools are being dealt out to winners (particularly in LoL and Dota2). In order to become a contender both require considerable amount of talent and dedication. In terms of action, both have an equally thrilling experience. The main difference is that professional gaming is still considered a niche among a more widespread audience and to understand their level of play, the individual audience must have fairly decent experience playing the game. In most conventional sports, the basic rules are fairly simple.

An example: In football (soccer for Americans) the basic strategy is to score a point against the opposing team and as the game progresses the purpose of each player is made clear simply by listening to the commentary (defenders, midfield, strikers, goalkeeper). In dota2, the goal is to defeat the opposing teams throne. Now to understand every Hero and their active item use WILL take time and can't be learned by simply watching a match. Furthermore map awareness and meta game counter picking is something that only comes after many games of dota2. Unfortunately, this will put many people off watching e-sports.