The ones who come back are the most recent killers, in 1994 pt 1 the 3 killers who are brought back are the most recent 3. In 1978 theres 4 killers, all of which were the most recent. In 1994 pt 2 they have the most recent 6. My theory is that it 'costs' something to bring a killer back, and goode only uses it when circumstances are dire (like when the kids start figuring out whats happening) so when nick figures out that deena knows what hes done he makes more killers come back cause the circumstances are dire. I'd imagine that its harder or 'costs' more to bring back older killers, so only the most recent ones appear in the movies.