The profitability of cartel price fixing in a slow, supply constrained market is enough carrot to cause to lead to anticompetitive coordination.
In this game theory setup, breaking from the cartel only pays off with excess supply (long vacancies, new units you could lease), which is fairly atypical especially bc of regulatory hurdles. If the market is supply constrained, being part of a cartel maximizes profits. Over longer periods, a competitor could increase supply and inject some dynamism that rewards breaking from the cartel, but for urban/suburban apartment complexes, this is long time and of limited effect to the cartel's pricing power.
This is why the FTC exists to prevent this type of illegal coordination (bc it's so attractive to profits!)
Edit: typo