Currently, miners are hitting the wall. Technology is the first problem. Most miners are already using the latest nodes, namely 28nm and 20nm processors offered by TSMC and GlobalFoundries, while the first 14nm/16nm FinFET ASICs are expected next year. Yet progress is slowing down due to a number technical limitations plaguing all chipmakers.
http://www.coindesk.com/bitcoin-mining-can-longer-ignore-moores-law/