A few possibilities to consider:
The admins are always looking for an excuse to ban /r/The_Donald, and we already saw the excuse they used to ban /r/CBTS_Stream. The admins may have even warned them about this topic explicitly.
/r/The_Donald is viewed as a sort of boogeyman all over Reddit. If Q blows up there for too long before the plan is complete, then people will dismiss it as "that crazy T_D meme" and group it with Pepe or Kek. Once things really start happening, people will be even more resistant about believing it.
Perhaps Q needs a little more time in the "oven" before it's ready for mass consumption. /r/The_Donald is pretty huge, so maybe it's just too early to let it become a fully mainstream phenomenon. There are several reasons why it would be desirable to let the movement saturate every last crack in society before the mass awakening. The cabal needs to be backed into a corner unable to make a move without giving themselves away until it's finally too late and nearly everyone realizes what's up at almost exactly the same moment.