The American Heritage Dictionary (4th ed., 2006) supplies a “Usage Note” after its entry for the adjective different, in which it is claimed that “Different from and different than are both common in British and American English.” An attempt is then made to fashion (out of whole cloth) a series of rationales to counter unnamed “language critics” who “since the 18th century . . . have singled out different than as incorrect, though it is well attested in the works of reputable writers.”
What is beyond dispute is that increasingly in contemporary speech and writing one finds the adjective different (and the adverb differently) taking the complement than instead of the normative from, e. g., Peirce is different than Wittgenstein, etc. In colloquial American English the construction with from appears to be diminishing in frequency, although there are certainly still many speakers who never use than instead of from after different(ly).
An explanation of this change in syntax can begin with a consideration of the two semantic relations involved in the concept of difference––distinction and comparison––together with the condition of their being ranked hierarchically vis-à-vis each other within the semantic syntagm (complex unit) that comprises this concept and its realizations. (Note, incidentally, that the semantic asymmetry that emanates from the ranking of the relations in a linguistic syntagm is not the same thing as asymmetric relations in logic.)
What we have here is an alternative ranking of the two relations embodied in the concepts of distinction and comparison. These two hierarchies reflect two possible emphases, which can be called hypotaxis and parataxis. The normative construction different from can be analyzed as a syntactic diagram (in the Peircean sense) of a hypotactic emphasis in the ranking of the two relevant components: distinction is ranked higher than comparison (i. e., the latter is dampened or bracketed in the syntagm). This ranking emphasizes the asymmetrical aspect of the terms juxtaposed or combined in the syntagm. The non-normative construction different than, on the other hand, can be analyzed as a syntactic diagram of a paratactic emphasis in the hierarchy: comparison is ranked higher than distinction (it is distinction that is now dampened or bracketed rather than comparison). This second ranking emphasizes the symmetrical aspect, that the terms are juxtaposed in the syntagm. Cf. the use of different predicatively (A is so different!) or appositively (A is a different product!––i. e., ‘distinctive’) in advertising lingo (but not only).
From the point of view of linguistic structure, then, one could conclude from this analysis that the ranking difference (alternative hierarchies) in the semantic syntagm associated with the concept and word different is manifested syntactically as a difference in complements: from means difference or non-equivalence, and than means comparison or equivalence of the terms on either side of the construction. Notionally, one could interpret this change as a shift away from the understanding (inherently, in the grammar) of different as embodying contradictory relations, to that of its embodying contrary relations (gradience).
An analysis that trades in competing semantic hierarchies may not seem to constitute an explanation of the change from one syntactic pattern to another, but this is not strictly so. The nature of grammar is such that what appears in speech or is expressed can always be traced to underlying grammatical relations ––which are semantic in their essence ––as its cause. But in the syntactic change discussed above, one unsatisfied with this type of intrinsic explanation might which to speculate about causes inherent in the larger communicative situation. Although hard evidence is unavailable, perhaps the change has its transcendent explanation in the larger tendency within contemporary American culture to neutralize social hierarchies, i. e., to scant hypotaxis in favor of parataxis. With the encompassing social structure and its flux as a reference point, the change in grammar would find its place as a piece of worldmaking.