Over and above everything else in Hubbard's writings, he was a pulp writer. (With all the limits and flaws of a pulp writer.)
For him, Science Fiction was just another type of pulp. A type that he only wrote later in his pulp career.
Now, some pulp writers transcended their pulp origins, many did not, and were left behind as the pulps died. (Think Doc Savage, for example. Lester Dent (and his ghosts) were mostly pulp writers, who disappeared (mostly) with the pulps. (A useful point of comparison)).
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