I would consider number of active users to be the primary factor in determining volume. This in itself is determined by trust i.e. how much each DEX is trusted. This is not merely a factor of how secure or decentralised it is, but rather of how well known and well marketed it is (not a usual DEX forte).

I think that we will see a major volume increase with time after a few things have happened:

• The market turns properly bullish again and people look for good new places to trade. Current trading overall is a fraction of what it once was at its peak.
• DEXs become more well known. Names like Switcheo are slowly creeping into the crypto vernacular. As they become more commonplace, so they will get used more.
• The big names in the industry will launch their DEXs. Here I am speaking specifically of Binance (once it decentralises) and NEX (once it launches). Both of those should be phenomenal in the impact they will have on the crypto world.