What, exactly, is the reason to keep any technology secret?

1) the utility of novel technology in war.

This is probably the most common motivation. It has allowed a fraction of humanity to be become an Empire. Or shining beacon of democracy with a thousand points of light, please feel free to use whichever description you find appropriate.

2) if the technological differential between a would-be ruling class and those it would enslave becomes great enough, the rulers can pretend to be gods.

This one is harder to pull off, since those who would rule tend to be inbred morons barely capable of drinking beer, watching the tube, and eating pretzels at the same time without choking. Such Aristocrats tend to look down on people who actually expend the effort to know about what they're doing.

[in reference to this]