Going back to the original question: What are the possible reasons why the list of 40 Principal Doctrines does not feature a statement explicitly stating pleasure to be the goal of life?
It is possible that there were more Doctrines than just 40? So that what we have from Diogenes Laetrius is incomplete?
It looks to me that Diogenes Laertius lists two goals -- and this excerpt shows a conflict. On one hand there is "health of the body and tranquility of the mind" but further down "pleasure is the alpha and omega".
Letter from Menoeceus (R.D Hicks):
QuoteDisplay MoreWe must also reflect that of desires some are
natural, others are groundless ; and that of the natural some are
necessary as well as natural, and some natural only. And of the
necessary desires some are necessary if we are to be happy, some if
the body is to be rid of uneasiness, some if we are even to live.
[128]
He who has a clear and certain understanding of these things will
direct every preference and aversion toward securing health of body
and tranquillity of mind, seeing that this is the sum and end of a
blessed life. For the end of all our actions is to be free from pain
and fear, and, when once we have attained all this, the tempest of the soul is laid ; seeing that the living creature has no need to go in search of something that is lacking,
nor to look for anything else by which the good of the soul and of
the body will be fulfilled. When we are pained because of the
absence of pleasure, then, and then only, do we feel the need of
pleasure. Wherefore we call pleasure the alpha and omega of a
blessed life.
[129]
Pleasure is our first and kindred good. It is the
starting-point of every choice and of every aversion, and to it we
come back, inasmuch as we make feeling the rule by which to judge of
every good thing. And since pleasure is our first and native good,
for that reason we do not choose every pleasure whatsoever, but
ofttimes pass over many pleasures when a greater annoyance ensues
from them.