why anatta is insufficient to uproot suffering? Here is a simple answer:
One type of suffering comes from craving. In other words, if I am sitting over here and in my mind I crave for something over there that I can't obtain - that very experience of painful mismatch between the two is suffering. Another type of suffering comes from fear of losing something. This is the same mechanism at play as with craving, just in reverse: I sit over here having something nice, feeling good - and I know that over there in the future I will inevitably lose it. Hence, that experience of the painful mismatch between the two is suffering.
Oftentimes (but not always!) these two scenarios occur with our sense of "I" as the object of craving or loss. In case of craving, let's say I want to become a great Buddhist teacher, or a successful businessman, or a big boss, or a father of a big happy family. The mismatch between what I want to be and what I am now is experienced as suffering. Or, in the case of a loss, let's say I already feel that my state of being is good, however what if I know that in the future, due to a sickness or war or simply due to old age - I will lose my happy peaceful state of being. Again, there is a mismatch in the mind, and this mismatch is experienced as suffering.
Anatta can help us become immune to this type of suffering. If we stop identifying ourselves with any social role or any lifestyle or even with this body or this mind - then, if we start losing any of it, we will not regret and we will not suffer. If we are not attached to any idea of "I", any particular manner of being, we will be at peace with who we are now and with whatever happens in the future.
However, there is another type of suffering that cannot be fixed with anatta! This type of suffering comes from attachment to forms other than oneself, forms that have nothing to do with "I". For example, consider the situation when your teenage son suddenly declares that he is a gay person (a homosexual). Because you are so attached to an idea that your son should be a proper man, there is a great mismatch in your mind between expectation and reality. Because of this mismatch you experience dukkha, suffering. This type of dukkha has nothing to do with your sense of "I", it is about your son. But because you have a fixed idea of how things are supposed to be, you suffer whenever there is a mismatch.
So anatta can't help with this type of suffering. But shunyata can. According to shunyata, an idea (a form) such as "man" is a stereotype or a reified generalization. In reality, there are all kinds of qualities: such as strength, rationality, joy, sense of beauty etc. - that are present in both man and woman to various degrees and should not be lumped into "man qualities" or "woman qualities". The example of "man" vs "gay" is just one example. Our everyday social life is full of examples when we attach to some form, some idea, some concept of how things are supposed to be, some idea of "rightness" that is based on some overgeneralization - and then from this attachment there comes all kind of conflict and suffering.
When you understand shunyata, you understand the principle behind all generalization, all delineation of entities. You clearly see that every human concept in this civilization is some kind of convention, abstraction, or generalization. You also see that our subjective perception, from our limited experience and single point of view, is always partial - it never includes everything that possibly exists. Because we don't know that our experience partial, we assume that our understanding of the world is how it really is - and then based on this we engage in action that leads to conflict and suffering. Deep understanding of shunyata uproots this source of trouble.