Why do transactions propagate?
It's true, there is a bit of a Tragedy of the Commons going on here. All miners want transaction to be propagated to them so they can include them and get the fees, but why would they expend their own bandwidth to help others hear about those transactions?

Fortunately, though, miners are not the only ones on the network. There are currently almost 6,000 reachable full nodes on the network, and a small subset of these are actually miners. So, even if all the miners stopped propagating transactions (they still have to propagate their solved blocks to get paid), information would still spread fairly quickly throughout the network.