Because "generic static object" wasn't a thing the cars looked for at all until just recently in the FSD beta, and that appears to be for road debris. If I had to take as guess as to why this particular object gives it trouble I'd guess because it's floating.
All the objects the beta now seems to be looking for are ground based implying it's looking for color oddities in the road area.
Remember all the fancy sensors and neural nets are still feeding into hard coded rules. If the car hasn't explicitly been told to look for something and how to respond to it, it might as well not exist as far as behavior is concerned. Which means that the solution is simply to make rail road barriers a specific object it can identify and create behaviors for it.
(As for why radar didn't cause it to emergency break it's likely because it was at low speeds, and radar has trouble with stationary objects.)
