I don't know that I would describe Suricata as "essentially open source Snort" since Snort itself is licensed under the GPLv2.
Both Suricata and Snort are GPLv2
(my personal speculation) Suricata was funded by DHS because Snort was used heavily within the US government and I think they were worried about having such a central tool being owned by a corporation . There were also some issues with how Snort did scaling to multiple CPUs (Snort 2 was single CPU bound, so to run on multiple CPUs, you'd have to start up an instance of snort per CPU). Snort 3 finally is out years later and has fixed this problem.
I'm also guessing that it partially was started when Sourcefire was about to be bought by Checkpoint systems back in 2006 (https://www.washingtonpost.com/archive/politics/2006/03/24/p...), which was SUPER close to happening but got killed for "reasons".
To peer comment: note that many personal installs of Suricata out there still use the VRT rule sets released by the Snort/Cisco teams. Rule sets are where Sorucefire made some of its money (though most was the hardware boxes they sell to corporations). Suricata never got in the business of publishing rulesets as far as I understand.