Expense ratios for stock and bond mutual funds fell in 2015 to the lowest level in at least 20 years. Unfortunately, not all funds passed along savings to their shareholders, said Todd Rosenbluth, director of ETF and mutual fund research at S&P Global Market Intelligence.

Rosenbluth said there are a number of factors that contributed to the 2015 decline, some of which are tied to the strong inflows into passive funds, which seek to match a market index instead of picking stocks. In 2015, seven of the top 10 mutual fund asset gatherers were passive products offered byVanguard, including four equity funds, according to Rosenbluth.

"As more money flowed into lower-cost passive funds, the overall expense ratio has come down," said Rosenbluth. "At the same time, many asset managers cut the expense ratios of their active funds."

According to S&P Global Market Intelligence research, 42% of the 897 equity mutual fund share classes with assets under management greater than $1 billion lowered their fees last year. As a group, the funds with reduced expense ratios attracted $13 billion in inflows. In contrast, the remaining funds that either kept expenses the same or raised them suffered collective outflows of $107 billion.

"Active funds continue to struggle against index based products when it comes to performance and that is not keeping assets in-house either," said Rosenbluth.

S&P Global Market Intelligence thinks that the new Department of Labor fiduciary standard rules will encourage advisers to pay more attention to a fund's expense ratio and to consider low-cost passive ETF alternatives such as iShares Core S&P Total US Stock Market (ITOT) - Get Report .

"Greater due diligence will be required by advisers and high-cost, underperforming funds will be harder to justify," said Rosenbluth.