Alright, here's the deal with the usage of type-reducing berries.
1) lack of exploration. Okay, there's been experimentation on things like YacheChomp, but have people actually TRIED type-reducing berries? I mean be honest here, in practice, are you going to go for the unexplored, less tested option, or are you going to go for defined, more tested options such as Life Orb and Leftovers? Most people will choose the latter. It's human nature, the thought process behind many of the things people do.
2) lack of use after they are consumed. I'll give you a situation. I've got a Tyranitar. I have two choices: Chople Berry or Leftovers. What am I going to ask myself? The obvious question is, why don't I just get a Fighting resist such as Landorus-T that I can use as a pivot to gain momentum, and gain the extremely helpful passive recovery from Leftovers that Tyranitar loves. The fact of the matter is, once you consume the item, you have nothing. If the super effective move still kills, oh well, guess what, you wasted your item slot. It was a non-factor. This, I believe, is the main reason that type-reducing berriesnare not used very much at all.
That's my two reasons. Feel free to dispute them.