There are only three plausible reasons. 1) There are flaws in the code that they don't want to reveal to the US (perhaps it doesn't work as they've claimed, or

There are only three plausible reasons. 1) There are flaws in the code that they don't want to reveal to the US (perhaps it doesn't work as they've claimed, or it's more fragile/easily defeated than currently thought). 2) If the code leaked (for any reason), it could open up weaknesses for an adversary. 3) There is something extraordinary in the software, to Israel's benefit, that they don't want anyone else to have. This seems less likely than the other two.