i always thought the traffic between the nodes is also encrypted!
Perhaps someone else will stop by and confirm for us, but I'm pretty sure it isn't.
is there any particular reason for not using a simple SSL encryption for this?
Extra load on the node with minimal benefit?
this may be a little far fetched but isn't there a possibility of a man in the middle attack?
That depends on what you mean by "man in the middle attack". it is a decentralized peer-to-peer network. You only ever connect directly to peers, and if someone were to try to get "in the middle" that would just mean that they are yet another peer. Your node doesn't trust ANYTHING that it receives from any peer. It verifies everything. Since every peer that you connect to is connected to other peers besides you, they are ALL essentially "in the middle" between you and the rest of the network. Anything that any node could do with the unencrypted data, they could also do with encrypted data (since it would be data that they could necessarily decrypt).
There are, however, isolation attacks. This would be possible regardless of whether the communications between you and your connected peers were encrypted. In an isolation attack, an attacker would make sure that they are running ALL of the nodes that your peer connects to. Then your ONLY view of the network is what they allow you to see. In this situation, they could send you one transaction directly (making you think you got paid with a valid unconfirmed transaction) and send a completely different transaction to the rest of the network (one that pays themselves instead of you). If they have enough hash power, they might even be able to solve a block or two to convince you that the transaction they sent you is confirming (while the rest of the network creates many more blocks without your transaction).
i don't know what an attacker can gain from doing this except messing with one node but that is the first thing that comes to my mind with an un-encrypted communication.
Like I said, since your node is communicating directly with the attacker's node, anything they could do with un-encrypted communication they could also do with encrypted communication.