Take your pick
1. It was at the cleaners
2. His wife reminded him how much she hates that costume because Venom once tormented her back when they were first married.
3. The EIC is drawing it and he wanted to draw the original
4. Aunt May actually woke up but then screamed at the sight of the black Spider-Man and lapsed back into a coma
5. They're hoping Peter David will notice and pick up a story about the switch in a Friendly Neighborhood Spider-Man special.
6. He was in the black costume? I thought he was just really dirty.
7. It ran away because of all the talks and rants this story has generated months before it even started.
I'm gonna go with 3.
8. Editor and Assistent Editor f*cked up and just forgot short term continuity.
Reminds me of the mistake of MJ's broken arm which was quickfixed in a later issue.