There is "Open" in its name. Author probably doesn't want anyone to create more closed source software using it.
Why is this downvoted?
Presumably because the merits of GPL vs MIT are discussed so often on HN that people don't want any more of that.
Maybe because that’s not the place where to ask that question. OP should open an issue on the project to ask them instead of the HN community (who probably don’t know better).
Asking "is there a particular reason" comes off as quite bitchy and dismissive.
Also: The creator probably has a reason why they chose the license. This comment pretents to be a question, when in fact it just expresses unhappieness with that license choice.
If you are curious why they chose the license, why not ask them?