Because it reduces the margin. Apple puts 2GB into the current model, for $999. You want 2GB more, it's another $100. (A bargain, IMO)
If they introduce the next model with 4GB of memory and start the price at $999 (or lower it, as many have discussed) they are losing money right off the top because their component cost has just increased, with no return for the upgrade.
While I would love to see the low-end MBA get 4GB of RAM as a stock configuration, I would not expect it.
With this philosophy computer companies would be offering 8megabytes of RAM with a $5000 4gb upgrade. The old luxuries become the new standard.