1. Change of behaviour between versions, (e.g. the way a frameworks integrate with a page)
2. Reliance on functionality being supported into the future
3. Customer expectations
4. Browser support
That said, it is true that a frameworks gives you the ability to put together an app in a few weeks. However, when you know that what deal with with something that needs to be maintained for years than knowing what does each piece of your code is far more important.