The reason is that "meaning exists only where there is a distinction between Intentional content and the form of its externalization and to ask for the meaning is to ask for an Intentional content that goes with the form of externalization" (ibid.
A different, more down-to-earth reason for reconsidering the mentalist paradigm came hand in hand with the flourishing of modern science and the consequent rise in popularity of philosophical naturalism and behaviorism.
The reason for this shift is that while we persist in seeing the quest for meanings as necessarily underlying and prior to any explanation of our language games, we are kept in the grip of a certain view of the nature of language – the view that a word comes to be meaningful only by being associated, within our mind, with some kind of entity.
Over and above this, he concludes that as we are not involving ourselves with any such esoteric stuff as pieces of mind, but only with the motions of parts of the material, tangible world, there is no reason to assume that to study, analyze and explain linguistic conduct necessitates any other tools or concepts than those which we already use to study, analyze and explain the rest of the world.
The reason is that to have an explicit rule we already need (a) language.
Thus, in my view, the concept of rule, far from being 'supernatural' itself, enables us to account for the specificity of human language, meaning, and reason, without invoking any 'supernatural' concepts.
However, there is no reason to abstain from making models of the semantics of language in the form of functions assigning expressions some kinds of objects10.
Although it is a matter of a relatively small collection of rules, they institute a space of chess games which is vast and incomprehensible not just for human reason, but as yet also for our most advanced computers.
It also seems to guard against the "bewitchment of our reason by language" (Wittgenstein) caused by words which are only seemingly meaningful: such questions as what does the word 'ether' stand for?
And it must be noted that I say of our reason, and not of our imagination or of our senses: thus, for example, although we very clearly see the sun, we ought not therefore to determine that it is only of the size which our sense of sight presents; and we may very distinctly imagine the head of a lion joined to the body of a goat, without being therefore shut up to the conclusion that a chimaera exists; for it is not a dictate of reason that what we thus see or imagine is in reality existent; but it plainly tells us that all our ideas or notions contain in them some truth; for otherwise it could not be that God, who is wholly perfect and veracious, should have placed them in us. And because our reasonings are never so clear or so complete during sleep as when we are awake, although sometimes the acts of our imagination are then as lively and distinct, if not more so than in our waking moments, reason further dictates that, since all our thoughts cannot be true because of our partial imperfection, those possessing truth must infallibly be found in the experience of our waking moments rather than in that of our dreams.