The person you quoted is in a sense "hedging his or her bets," so to speak. Instead of coming right out and saying,
"All critics agree that Melville is Bartleby,"
s/he softens the declarative sentence with the use of the word historicist (since I assume there are critics who are not historicists) and with the passive mode, so as not to be accused of being dogmatic or being unaware there are dissenting, non-historicist critics who believe firmly that Melville is not Bartleby.
Evidently, if what the writer is saying (and how he says it) is true, then perhaps the use of the passive mode is a safe "bet."
Here's an illustration of a little bet hedging of my own. Instead of saying
"Historians believe Obama's presidency is feckless in many crucial areas of public policy,"
"Historicists are inclined, even now, towards the explanation that Barack Obama's presidency is perceived by many as feckless in many crucial arenas of public policy."