I see, thanks for the through answers so far. However, I know fallout has no destructive environment features. But wouldn't it just be as simple as firing a rocket at the dam and poking a hole in it - rendering it useless? Caesars Legion seem obnoxious enough to do that, I don't see how it can be so Impenetrable. Potentially the main fighting point could be won/lost in seconds.
I'm not sure if anyone else has already mentioned this, but aside from the fact the dam is huge, there's also the fact that the Caesar's Legion wants control of it for
themselves. For the same reasons NCR does(energy, mainly from what I've read/heard/seen, but also water). If they destroyed it, that'd ruin the whole purpose of fighting with the NCR-when it comes to the Hoover Dam, anyways.
Any further details on the factions' reasoning are among the things I've forgotten from the past months of news, OR has simply not been told/leaked/covered yet. In which case you'd have to get the game and play it, or wait for others to play it and then make news of it.