To be honest, two weeks is longer than I would have expected a response in. They’re a brand trying to run a business and I’m sure they get a lot of people who just don’t respond back. If you want to be taken seriously as a business./brand ambassador, responses need to go back in a timely manner. There’s not set time, but I would say a reply within a few days at least. After two weeks, I would have assumed you weren’t interested myself! I don’t think this was a betrayal, as much as them moving on because they hadn’t heard back.
Now that being said, just because another blogger guest blogged for them doesn’t mean they wouldn’t ALSO want you to do it. The professional thing to do would be to send a follow up note, just making sure they saw your response, suggesting a few ideas for the perspective you could bring to the post and letting them know you would still be interested in working with them. Even if now isn’t a good time, you hope they’ll keep you in mind for the future.
Since they probably have the impression you either weren’t that interested or that you got back to them as an after thought (which obviously isn’t the case) the onus would be on you to now show them you are interested – or you can chalk it up to a good learning experience and work on other new opportunities. I guess it depends on how much you want to work with this particular brand.