Why normals can't skip.... Well, Blizzard chose the raids to be linear. As to why that is, your guess is as good as mine. Ulduar and Icecrown were a lot less linear. But, as a side-effect, all non-linear bosses will need to be equal in difficulty, because they cannot expect the common, casual raider who steps in there to do the research, or have the insight, that heroic raiders need. I guess it's just easier to make them linear for that reason, and perhaps this is why Ulduar, and even (though less so, especially with the nerfs later on), ICC was so brutally unforgiving to more casual groups.