The existence of barriers to entry (patents, rights, etc.). The most attractive segment is one in which entry barriers are high and exit barriers are low. Few new firms can enter and non-performing firms can exit easily.
Government policy
Capital requirements
Absolute cost
Cost disadvantages independent of size
Economies of scale
Economies of product differences
Product differentiation
Brand equity
Switching costs or sunk costs
Expected retaliation
Access to distribution
Customer loyalty to established brands
Industry profitability (the more profitable the industry the more attractive it will be to new competitors)