Why can current only flow in loops?
What you have to understand is that electrons don't move on their own but as a chain... like a bunch of kindergarten kids tied together hand in hand.

Consider the following drawing of a series of balls in a track system.



It is fairly obvious that you can use your finger to push the chain of balls around either loop and they will move freely.

However, you can NOT push any balls across the joining trough at the bottom because there is nowhere for the ball to go.

That's what also happens in wires. If you DID manage to force an electron into the right loop, perhaps using an inductive coil or something, there would be charge difference generated between the two loops which would quickly force the electron back once you took the force away.