Question: Why does anybody (including Sam Raimi) claim that Evil Dead 2 is a sequel with flashbacks to the first film? As a sequel, there is virtually zero continuity to the first film. This was an obvious remake.
Chosen answer: Evil Dead II starts out with a recap of The Evil Dead where Ash cuts off the head of his girlfriend Linda with a shovel and then is attacked by the entity, which is how the first film ended. While Linda is played by a different actress, it's still meant to be the same events as The Evil Dead. The cabin windows are still barricaded from the end of the first Evil Dead, and Ash's pant leg is still torn from Deadite Scott attacking Ash. Additionally, the end credits say: "Evil Dead II", the sequel to the ultimate experience in grueling horror, was filmed in Wadesboro, North Carolina and Detroit, U.S.A.
Answer: Sam Rami could not get the footage of Evil Dead to use in the sequel, so he remade the movie in a shortened form for the beginning. Since Linda was the only original character other than Ash to come back he included her in the recap but deleted the others as not to waste time. An in universe answer could be that trauma of losing his friends made Ash block them out and rewrite history in his head, except for Linda who comes back to haunt him in the movie so he is forced to deal with it.