theory is that the lower price, the less cost of trading,
if the trading funds is fixed, the price is lower, the volume is more.
e.g.
$1000 can trade 10 times at $100/BTC, but it only trade 5 times at $200/BTC
It seems to be the truth, but it almost no trading volume when Bitcoin in 2012-2013, and the price only a few dollars that time.