Contactless cards rely on electrical signals transmitted through the contactless reader near the cardholder's terminal to authorize a purchase. These signals can be interrupted by other nearby payments, cellular reception, or even metal objects such as coins.
Apple Pay relies on Magnetic Secure Transmission (MST), which employs a strong magnetic field and unique data encoding scheme to ensure secure transactions without the need for contact. However, some older contactless readers do not support MST technology, so Apple Pay cannot be used with those cards.