The decreasing number of miners in the bitcoin ecosystem is a plausible explanation for the decrease in the bitcoin mining difficulty. (coinfox.info)
The reason for this is that there are a finite number of miners processing each block as well as there are finite number of transactions that can be included in a block. (captainaltcoin.com)
More and more number of miners are entering into the bitcoin mining network and hence competition increases. (kryptomoney.com)