That's a really good question. Consider:
1) Jobs originally said that including WiFi, made up for lack of 3G. If he keeps with that logic, then WiFi shouldn't be necessary, and might even be seen by him as "impure"...
2) Cost is a factor. It's always cheaper to leave out a radio. Make it easier for engineering to add something else, too... like GPS or double the memory chips. Hmmm. 32GB?
3) Battery... what can you say? WiFi chews it up, if left on. A 3G phone will last much longer without it possibly being turned on... a don't-let-the-user-decide Apple kind of thing.
Many EVDO phones come without WiFi, and owners don't miss it that much. (I would, just for Slingbox around the house.)