Hangouts Chat bots is only sent new messages. Edited messages don't get sent to the bot. Though as Brett mentioned you can submit a feature request if you can think of use cases for the functionality (see below).
I suggest checking the Hangouts Chat API support page and search whether someone's filed a feature request (FR) on Google's issue tracker. If so, star it and add a link to it here via a comment. If not, file a new feature request and also link it here so others can vote on it.
As for the purpose of @mentioning a bot in rooms, this is likely done for privacy/security purposes. Having a bot or bots always listening to conversations in a room could raise such concerns. I'd expect bots to only get messages when it is directly @mentioned in a room unless Google feels otherwise. Who knows... may be if enough people star it, they may consider it.