Maybe they are crippled DPS jobs because their role is not just about DPS but other things like tanking and healing.
If tanks had positionals, they would have to spin the boss a lot and annoy the DPS. Tanks needing to keep up something is not good either.
Dark knights had a lot to maintain in Heavensward because they had to keep their MP above 0 to stop it expiring and use attacks but not too much, with multiple combos and a bloat of off-globals, a standalone damage over time to maintain and a lot of situational abilities that made you have to think. It was so incredibly complex to do it right without losing everything. Although I somehow managed to do it all correctly while still tanking I really do not believe a tank needs to be that complicated. It's unnecessary and makes it harder for people to get into tanking, in what was an even less popular role in Heavensward.