From an evolutionary standpoint, there is no reason why it should remain erect for long after ejacuation. Mother nature probably thought that it would be more productive to get back to work...

But yeah, it's normal to lose an erection within a short time after ejaculating. Personally, it seems like I lose most of my interest in sex after ejaculating and I can't remember ever maintaining a full erection for more than a minute or two after cumming. Usually, the angle starts going down within the first 15-30 seconds...