TJSON requires arrays be homogenous, and this is presently accomplished by specifying the types of arrays up-front in object member names and rejecting the array if any of the contents don't adhere to the type signature.
With toplevel arrays, in absence of this type information being explicitly specified in an object, implementations would have to rely on detecting homogeneity at decode time.
This is certainly possible, and in fact the serialization logic does it. But it seems like a sharp edge to include in deserialization logic in a security-oriented format. The format aims to keep the deserialization logic free of any sort of "guesswork".
