Have you check the mergecap man page ?
The output file frame encapsulation type is set to the type of the input files if all input files have the same type. If not all of the input files have the same frame encapsulation type, the output file type is set to WTAP_ENCAP_PER_PACKET. Note that some capture file formats, most notably pcap, do not currently support WTAP_ENCAP_PER_PACKET. This combination will cause the output file creation to fail.

Have you check checked the mergecap man page ?
The output file frame encapsulation type is set to the type of the input files if all input files have the same type. If not all of the input files have the same frame encapsulation type, the output file type is set to WTAP_ENCAP_PER_PACKET. Note that some capture file formats, most notably pcap, do not currently support WTAP_ENCAP_PER_PACKET. This combination will cause the output file creation to fail.