This sentence compares two clauses with the use of the conjunction "THAN". The first clause is "Inuits of the Bering Sea were isolated from contact with Europeans longer." The second side of the parallel structure must also be a clause. Thus it needs the verb "were." The structure might sound a bit odd because the sentence uses INVERSION - the verb "were" stands before the subject "Aleuts or Inuits of the North Pacific and northern Alaska." Also if we don't include the verb, the sentence becomes ambiguous.

Before "than" it is optional to use inversion.