With other copper blocks, you can space them out to oxidize at a faster rate. But since the copper door is a 2 block structure, it sees the other half of itself as another distinct copper block, which significantly slows the oxidation rate.

Steps to reproduce:

1. Place a bunch of copper blocks in a grid with 4 block spacing between
2. Place a bunch of doors in a similar grid
3. Wait and watch oxidation rates.

Observed: doors oxidize much slower

Expected: they'd oxidized at similar rates, since they're spaced out the same.