They did...two years later. They obviously had the tech for spirit->hit in Wrath, since Agi->dodge had long existed, but they had not hit upon the idea yet.
I wouldn't say there was a gap at the BC. Armor-to-class distribution was 3-2-2-2. Plate wasn't lacking; cloth stood out. Five years later, 3-3-2-3 looks like a gap, but I don't believe that alone will dictate Blizzard's next class developments. Thus far, each expansion has had made class changes connected with the expansion, and we should fully expect this to continue. If they want to add tinkers, there will be story reason to do so. If they want demon hunters, that will make thematic sense as well.However, if you look at past implementation, you notice that Blizzard focuses on the large gaps, not the tiny ones. The tiny ones can be fixed by simple stat manipulation. Large gaps are filled by new classes.
BTW, I don't think tinkers should wear mail. Messing with machinery and electricity, the last thing you want to do is wrap your body in conductive materials. Although that does sound appropriate for WoW's goblins to do.