Hi,
Because there is a difference between packet-sniffer and packet-analyzer .
https://ask.wireshark.org/questions/6737/packet-analyzer-vs-packet-sniffer
You can always use one of those two options to capture to PCAP file :
1. in the shell , use the tcpdump with the (-w) option to save to pcap file
for example:
% tcpdump -i fe-0/0/1 -s 1514 -w DHCPSRX.pcap
2. you can configure filter capture " term sample accept"
http://kb.juniper.net/InfoCenter/index?page=content&id=KB11709&actp=search
You can use the datapath to capture to PCAP , that would be relevant on high-end ..
http://kb.juniper.net/InfoCenter/index?page=content&id=KB21563&smlogin=true&actp=search
AFAIK, Those are the only ways to capture .. there is no show security flow traceoptions | save abc.pcap