The fact that e-sports has exploded I think has a lot to do with how they've become more similar to real-world sports. We've moved from very artificial e-sports like Starcraft, fighting, racing (usually 1v1 games with extreme focus), to distinct role-based "MOBA" games like Overwatch, League of Legends that are much more similar to popular existing games like soccer and baseball.
Unlike in the earlier games, they rely slightly less than pure mechanical skill, which makes the sports accessible. They're team games with differentiated roles, allowing beginners a way into the game playing with, rather than against their friends. Sounds familiar? This could describe soccer, basketball, football, and all of the top spectator sports today.
Their only real substantial difference is that they're far more accessible, requiring only equipment that many already have (the games intentionally minimize system requirements) and don't require real physical athleticism.
I think it's no surprise they're becoming popular as spectator sports: they're building on a proven formula, but adding a dash of accessibility.