It's Game Theory problem. If you are getting more value out of the system by maintaining it in the long-run, it would make no sense to attack it and destroy its value. However, once you can extract more value in the short-term through the attack than by being a long-term participant, it becomes attractive.

With BTC's block reward continually being reduced, TX fees will have to increase in order to avoid reaching the point where large miners could become tempted to attack the network.