The appendix to Verhoeven's metallurgy for bladesmiths goes into this in rather
gory and hard to understand detail. As simply as I remember:
While the carbon content is down around 440A, the chromium is also down. The
stain resistance of a steel is based on chromium not tied up in carbides. The
completeness and thus potential hardness of the martensite matrix in steel is
based on carbon in the matrix, not carbon in carbides. If you have too much
chromium and carbon the two happily make (not so little) carbides together, thus
producing a blade that is neither as hard, nor as stain resistant, nor as fine grained
as a blade where the two elements are in the optimum balance.
In other words, the synergy of the chemistry works out better in AEB-L (13C26 --
12C27 is similar with less carbon).