At the time (still?) it seemed impossible to bridge Discord channels (which are viewed very much as 'create as you please') to Matrix rooms without painfully doing it one-by-one via a config file and managing/creating both on each side (Matrix really seems stuck on the idea of preferring its 'one room' model and has not really embraced channels in the Discord sense.) Discord had also just gotten forums with no way to bridge them.
Additionally, our Matrix rooms were seeing a good amount of spam that Discord was not, which would then get relayed to Discord via the bridge and would require moderating in both sides.
Finally, our strongest Matrix advocates firmly disliked the idea of a bridge because it meant their Matrix messages would go to the proprietary Discord company.
In the end, we let them run side-by-side for a little while then the Matrix quickly died.