From Erik Lauer:

The approach we have been taking with reanimator strategies, is to allow great reanimation targets, but to usually charge 5 or more mana for reanimation spells. This gives us a lot of design space for big creatures that you’ll actually cast as well. As far as older formats go, Griselbrand sets the bar very high.

From Ian Duke:

We also designed the new Eldrazi to have a lot of their power contained in their cast trigger. Between that, and them not having annihilator this time around, we were okay allowing them as reanimation targets. And by not having to write out the anti-reanimation text that the original Eldrazi titans had, it gave us more room for other cool abilities.