Except he didn't say it[1], or at least no-one who was there remembers him saying it, and it seems like it's something he added in a later revision of his own memory.
As John von Neumann later said, 'sometimes someone confesses a sin in order to take credit for it'. It's clearly what Oppenheimer wanted.
[1]https://en.wikipedia.org/wiki/J._Robert_Oppenheimer#Trinity