The riddle of the mob tower

http://imgur.com/a/PYbhW

The riddle of this mob tower is that for some reason no mobs spawn in it. This essentially makes for a very ineffective mob tower. I was wondering if a kind redditor could help explain to me why it won't work.

If somebody can help me and if somebody else is curious about how it was made I would gladly show.

Thank you very much reddit.