Why was Celsus historicist?
I think that he was historicist for two reasons:
1) the Christians of the his time did insist on a literalist reading of the Gospels.
2) the Judea of the his time was full of inspired prophets.
The Celsus's argument to believe in a historical Jesus:
If the Judea was full of inspired prophets, and the Gospels describe an inspired prophet, then Jesus existed and was an inspired prophet, too.
While the point 1 is recognized by all, maybe it is not clear the second point.
So Celsus's Jew described the Judea of the his time (II CE) regarding the inspired prophets:
(VII, 9)"There are many," he says, "who, although of no name, with the greatest facility and on the slightest occasion, whether within or without temples, assume the motions and gestures of inspired persons; while others do it in cities or among armies, for the purpose of attracting attention and exciting surprise. These are accustomed to say, each for himself, 'I am God; I am the Son of God; or, I am the Divine Spirit; I have come because the world is perishing, and you, O men, are perishing for your iniquities. But I wish to save you, and you shall see me returning again with heavenly power. Blessed is he who now does me homage. On all the rest I will send down eternal fire, both on cities and on countries. And those who know not the punishments which await them shall repent and grieve in vain; while those who are faithful to me I will preserve eternally.'" Then he goes on to say: "To these promises are added strange, fanatical, and quite unintelligible words, of which no rational person can find the meaning: for so dark are they, as to have no meaning at all; but they give occasion to every fool or impostor to apply them to suit his own purposes."
Note that Celsus isn't describing a zealot kind of Messiahs, nor simply apocalyptic Messiahs. He is describing Messiahs à la Simon Magus: people possessed by God/the spirit of God and imitators/emulators of the deity.
Therefore Celsus believes mistakenly that the first-century CE Judea was full of inspired prophets of this kind just as it was the Judea of the second century CE.
But Josephus wrote only about riotous messianists. Josephus didn't write about ''inspired prophets'' à la Simon Magus.
MY POINT is that in the Judea of I CE there were not prophets ''accustomed to say, each for himself, 'I am God; I am the Son of God; or, I am the Divine Spirit..''
If the first Gospel was written in the II CE, then ''Mark'' would probably be inspired by these II CE prophets who identified themselves with the deity. What moved ''Mark'' to euhemerize Jesus on the earth, was precisely what moved Celsus to believe that Jesus was historical: the presence of a lot of inspired/claimed ''Sons of God''.