Amap was innovative – the first tool to perform application protocol detection. Then a better approach was implemented into nmap, this and the large user base of nmap made amap obsolete. The reason for this is that nmap still does not have a good IPv6 support, e.g. UDP port scanning is not possible. The v5.3 release in April 2011 that enhances amap to perform better UDP IPv6 supports (before only application fingerprinting did work here, now the port scanning feature works too).
Source: https://thehackernews.com/2011/04/thc-amap-v53-application-protocol.html