As far as I know, they didn't because they though having a restaurant was a better idea at the time. Disneyland had a tiny walk through, WDW would have a huge grand restaurant. Of course, hardly any of the visitors that come to the Kingdom actually get to eat at or see that restaurant so I'm not sure if I agree with the logic. (Although a lot of the logic used in designing the original MK is a bit odd -that's a different topic though) Given how large the castle is I'm fairly confident that with some creative rearranging a walk through similar to California's could be created in and around the outside turrets and perhaps a little bit of the restaurant space. Disneyland's castle wasn't designed for a walkthrough either. However, given the low capacity it would have and modern ADA regulations I don't think there's a chance of it ever actually happening. The only way it would ever really happen is if they removed CRT and I don't see that happening.
