We incels can keep on harping that this is just due to looks, but its more than that. What has fundamentally changed about relationships is that they're now seen by women as platforms for consumerism and indulgence, rather than seeing it as something that enables people to build stability, security, and safety through long term loyalty and devotion despite challenging times which threaten to destroy that security.
Stability and safety aren't the priority for women anymore; pleasure is, so anytime a relationship is tested and goes through hard times of struggle, the woman immediately seeks pleasure elsewhere because she sees her relationship has an inexhaustibly stocked vending machine. When the vending machine can't pop out your snack, its time to look for another machine.