So Why did we fork
The OPNsense® developers have participated for years to pfSense® project but, in 2014, motivated by a desire of wanting to make a number of things differently, they decided to create their own project that reflects better their needs.
The stated reasons which led to the fork are mainly technical, but also due to security and code quality. In Last (but not least), the fork was due to the license change done by pfSense®, which caused some disappointment whitin the community.
If you wish to have further details on the reasons of the fork, please refer to the links:
https://docs.opnsense.org/fork/thefork.html#so-why-did-we-fork
https://m.reddit.com/r/PFSENSE/comments/3rh9dw/pfsense_vs_opnsense/