Because using a chain is too mainstream

Hipster bike.

What is that a train thing

Keep your feet on the pedals no matter what! (My autocorrect is all messed up)

Gear driven is always more reliable than chain driven.

It's a pretty cool concept but can you imagine the effort required to get the damn thing moving! The gearing ratio looks like it's 1:1 and as whospikedthepunch said, God help you if your foot slips off the pedal whilst you are moving, it will take your leg off!

Diff on a bicycle