Suppose all the miners form a cartel. They will have no problem funding themselves; they can all agree not to include any tx that doesn't pay high fees. The users would pay this fee because they have no other choice.
Some users will try to pay a fee lower than the cartel's threshold. One miner decides to defect from the cartel; he includes in his block all these low-fee transactions. This costs him nothing, so this is a net profit for him (he benefits).
Seeing this, users will know that even if they don't pay the cartel's high fees, they can still get their tx included eventually. Thus, their willingness to pay high fees is lower (that is, the miner consumed their willingness). Thus, more users will try to pay low fees, and the total revenue of all miners decreases.
But it's not just the one miner. Every miner will, individually, have an incentive to include low-fee txs. This means that even with a low fee, it's easy to get a tx to the block. Thus, no user will want to pay high fees, and the total revenue of miners will be low (this is the tragedy - for the miners, and due to the effect on network health, for all Bitcoin users).
This is completely analogous to the classical instance of tragedy of the commons, where all herders would benefit if they all grazed just their fair share, but everyone is incentivized to defect and overgraze, depleting the resource and causing everyone to suffer.
As long as the cartel has more than 50% of the hash power, it can dictate the terms and ensure any blocks not conforming to the cartel's fee policy would be orphaned. A cartel operating in such a fashion can prevent the tragedy of the commons you describe. I prefer to think of it as a race-to-the-bottom issue, with the bottom being inadequate network security. Realistically I would expect the 'willing' participants in the cartel to have somewhere between 70% and 90% of the hash power. The remaining fraction of hash power would also contribute hashing to the cartel because it would NOT be in their best interest to have their blocks constantly orphaned. Those miners might not like the cartel policies, but they would have little choice.
I wrote about this concept extensively in my earlier posts on this same thread.