This is because the GCMP256 cipher is the only one that supports both WPA2 and WPA3 with the Personal security type. The GCMP256 cipher provides stronger encryption and authentication than the CCMP ciphers, which are only compatible with WPA2. The source of this answer is the Cisco ENCOR v1.1 course, module 7, lesson 7.2: Implementing WPA2 and WPA3.