L.R. Palmer (1906-1984), The Latin Language
(1954; rpt. Norman: University of Oklahoma Press, 1988), p. 128:
Similar sensitivity to the tone of a word may explain why Caesar prefers non modo, non solum to the non tantum favoured by those who completed his work, tantum being ambiguous. It has been pointed out, too, that quomodo and quamquam are avoided by Caesar, although the latter occurs four times in Hirtius' Book 8 of the de bello gallico. Caesar, again, shows a preference for priusquam as against antequam and for posteaquam as against postquam. Differences of tone, vulgarism, and urbanity may account for many of these subtleties but, as Marouzeau suggests in his discussion of these facts, we should not ignore the factor of personal choice and sheer verbal habit. Why should Caesar never use quando or mox, and almost totally neglect igitur in favour of quare and itaque? Why his preference for timeo as against vereor and metuo? As for habit, the curious tendency for a word once activated to recur is illustrated by Caesar's use of the rare phrase e regione no fewer than seven times in the seventh book of the Gallic War although only one other example is found in the rest of that corpus.
See H. Merguet, Lexikon zu den Schriften Cäsars und seiner Fortsetzer, mit Angabe sämmtlicher Stellen
(Jena: Gustav Fischer, 1886).