A few points:
a) This is a question of resources. We could either get all the classes in and not really have them have much of their original flavor, not be well tested, each class has a single suit of armor appropriate to them, etc, or we could get some smaller subset in of a higher quality level and build on them. Of course we'd like to have more classes in, but time and resource wise it is not feasible.
b) My hope is that we can get Sorcerer, Barbarian, Paladin, and Bard in by the end of OE. They are relatively easy to add compared to Druid, Monk, etc, as most of their abilities are either not too difficult technologically or are combinations of other classes. Ranger, Druid, and Monk either involve a lot more tech (Animal Companion AI, wildshape), or a lot of custom animations (Monk combat) if we want to do them right. We're not going to do them unless we can do them right, as what's the point of playing a druid if you can't wildshape and don't have an animal companion?
c) The difference in terms of skills you buy between one of the four main classes and the other classes for the most part are a minority of your XP. For example, if you want to play a Ranger, day 1 of EE you make a Fighter and the XP you spend on Base Attack Bonus, Fortitude saves, Hit Points, special attacks, etc, is all going to be just as useful for a Ranger. If you want to play a ranged focused Ranger, just build an archer Fighter and you're about 80% of the way there. Once Ranger gets made available you could be up to speed on being a Ranger pretty quickly (around 20-30% of the time you spent buying Fighter). The same is very true for Paladin (you can almost build a Paladin day one of EE from mixing Cleric and Fighter abilities, all you're missing is some class features), Barbarian, and Bard. Sorcerer is a little trickier due to spell slots, but still not difficult.
