Its no surprise that mining power is concentrated[1]. Activities across populations tend to have a power law distribution[2]

[1] https://www.blocktrail.com/BTC (scroll to "Pool Distribution", today more than half the mining capacity is in two pools)

[2] https://en.wikipedia.org/wiki/Rank-size_distribution:

"The rank-size rule (or law), describes the remarkable regularity in many phenomena, including the distribution of city sizes, the sizes of businesses, the sizes of particles (such as sand), the lengths of rivers, the frequencies of word usage, and wealth among individuals. All are real-world observations that follow power laws"