That's not a promise to send any help or to help militarily.
It just says that if Ukraine is nuked (quite an extreme scenario to start with) they'll ask the Security Council (where Russia has a veto) to do something about it. You can argue that this constitutes a promise to 'help', but frankly sending a "get well soon" card would be more helpful.
Hence my first comment: The US and the West do not want to be forced to intervene and thus they never made any promise, binding or otherwise, to.
The UN did vote a resolution on Ukraine and Crimea in 2014.
As I also wrote, it is Russia that breached the memorandum but not respecting Ukraine's borders as they were but, tough.