The series was conceived by Marvel Comics' Editor-in-chief Jim Shooter:
Kenner had licensed the DC Heroes. Mattel had He-Man, but wanted to hedge in case superheroes became the next big fad. They were interested in Marvel's characters, but only if we staged a publishing event that would get a lot of attention, and they could build a theme around. Fans, especially young fans often suggested to me 'one big story with all the heroes and all the villains in it', so I proposed that. It flew. Mattel thought that kids responded well to the word "secret", so after a couple of working names bit the dust, we called the story 'Marvel Super Heroes Secret Wars'.'
Shooter also explained:
We went through a number of ideas for names for the toy line and series. Mattel’s focus group tests indicated that kids reacted positively to the words “wars” and “secret.” Okay. / Mattel had a number of other requirements. Doctor Doom, they said, looked too medieval. His armor would have to be made more high-tech. So would Iron Man’s, because their focus groups indicated that kids reacted positively...etc. Okay. / They also said there had to be new fortresses, vehicles and weapons because they wanted playsets, higher price point merchandise and additional play value. Okay. / When time came to actually do it, I realized that only I could write it.
sorry to be offtopic