Although bitcoin cash has a shared history with BTC, it didn't remove anything during the fork. Normally in any split, the remaining entity is lessened (as in a share split), which is why the value decreases. In this case, bitcoin cash is more of a clone than a split bitcoin. So it did not remove any value from bitcoin when it split.

Additionally I think there was a lot of money waiting to flood into bitcoin but the fork put them off, so they waited on the sideline until the fork was completed, then piled in.