Someone say iax is better then sip, but why we still use sip as VOIP protocol

what is the advantages sip has over iax, when iax is better than sip?

Lack of support outside the Asterisk community. I only know of a handful of carriers selling IAX trunks. Their users enjoy lower bandwidth usage, more stable connections, lower security footprint and no NAT issues.

It’s too bad the IAX is not supported more broadly.

I am a huge IAX (pronounced eeeecks) supporter.

if iax is much better why it is not support broadly? what advantages make sip enjoys popularity nowadays?

I thought I already answered that. IAX is an Asterisk standard. For one thing it is not an IETF standard.

Just because something is popular doesn’t mean it is the best. Remember the beta vs VHS. SIP is easy to implement, broadly supported and widely documented. It reached critical mass and it is what it is.

Heck I could make a good argument as to why MGCP is better than SIP.

Keep in mind, the proprietary vendors all moved away from SIP, IE: Cisco with SCCP, Mitel with minet etc.