Because Suite A are a set of non public algorithms that are used to encrypt data.
Them being non public makes it harder to workout how to decrypt the data.
They are also likely protected against a number of attacks that aren't public, even if these attacks aren't feasible against current algorithms.
For reference, see DES, where the NSA adjusted the algorithm to protect from a not publicly understood differential cryptanalysis attack. Many people claimed that the adjustment by the NSA was clear backdooring, though we know that was not true.
It was however purposely deficient in the length of its key, allegedly because "it was good enough" and for export reasons, but also because the NSA considered it easy enough to brute force.