A couple background info:
For people outside of Go community, Ke Jie is the new star of Go, currently Jie's Elo rating is #1, http://www.goratings.org/, Lee Sedol is #4. Media touted Lee Sedol as the greatest player of past decade, it was true couple years ago but Lee Sedol has been clearly in decline in recent years (as seen in the Elo rating).
2) Ke Jie is 19, at the peak age of his game. Lee Sedol is 33 and is seemingly in decline in his game, hence the 2:8 record vs. Ke Jie.
3) Ke Jie probably wanted to be challenged since he is the current world's #1 Go player. Sure there aren't much differences in terms of skill levels between top Pro 9 dan players but challenging a current world's #4 player is different from challenging a current world #1 player, isn't it?
It's a shame that the Western world doesn't know Ke Jie well(as his wikipedia page has very limited info and his award records.)[1] But these are moot points. After the 2nd defeat, it's increasingly clear that AlphaGo will defeat human player - it's a matter of time or a matter of a couple more games.
[1] https://en.wikipedia.org/wiki/Ke_Jie