I think one of the reasons is the implicit cost of a commitment to maintenance and a specific API that comes with standard library version being tied to Julia. See this summary here.
While arguably some functionality could be moved from StatsBase to Statistics, it is unlikely that the API for the whole package is that finalized and/or belongs in a standard library. OTOH, there is no pressing need to move anything, after all there is a package and people can just use it.