SNMP has many limitations and there were several attempts to replace it with new protocols. So it is not surprising to see the creation of OpenFlow, though I am not familiar with its origin and history at all.
There are a few advantages if we simply read the introduction,
http://en.wikipedia.org/wiki/OpenFlow
- TCP-based and TLS ready. SNMP is UDP based and TCP capable (a draft), but it does not include TLS support IMHO.
- Already supported by many vendors and many devices. SNMP was popular due to the same reason. If a new standard can achieve popularity among device vendors, it can compete with SNMP fairly.
- Grow up with SDN's demands. Remember that SNMP was designed in 1980s, and the Internet was not mature yet at that time, and nobody knew the complexity of Web 2.0 and SDN at that moment.