It's actually the exact opposite. People look for the path of least resistance. In the current situation, tanking without diseases generates more threat in less time amongst packs of trash (and possibly bosses as well); since that is the easier way, that is what people gravitate towards.

For the devs, they need to make sure that they're not fighting human nature in their design choices - that is always a losing battle. Outbreak is one great tool; another would be to find a way to boost early threat for DKs so they can keep pace with DPS. If we can hold onto aggro long enough to put up the diseases, I believe most players would do so. As it currently stands, however, I am getting aggro ripped from me before I can Pestilence the group. DnD > BB > RS/DS eliminates that. Why wouldn't I go that way?

If they do truly want diseases to be at the core of Death Knight play style, they need to find a way to make diseases matter to tanks. The damage it does and the effects they have just aren't enough. Perhaps a tank-only disease that has an increasing threat-over-time effect, or one that siphons health to the tank.