Why are collusions and cartels often unstable?

Why are collusions and cartels often unstable?

1. Name some different forms of imperfect competition.
2. Define Nash equilibrium. Why do firms in oligopoly situations reach Nash equilibria?
3. Why are collusions and cartels often unstable?
4. What is the market equilibrium in Bertrand competition with identical goods?

Why are collusions and cartels often unstable?