But the explosion in ETF's popularity has yet to influence the defined contribution market, according to Frank Polfrone, senior vice president of Access Data, the research arm of Broadridge that tracks 95 percent of all ETF assets.

Retail investors now hold 62 percent of all ETF assets. Polfrone says much of the retail growth is attributable to fee-based RIAs.

"More retail investors are turning to advisors to manage their money, and more advisors are using ETFs," he explained.

Increasing scrutiny of fees has made ETFs naturally attractive to advisors and investors, said Polfrone in an interview with Benefitspro. As investors have become more cost conscious, ETF product offerings have expanded to include virtually every investment category.

Polfrone thinks wider adoption of ETFs in 401(k) plans could be in the offing.

"ETF usage is likely to increase as more record keepers begin to support their use. And we've already seen Schwab launch a 401(k) offering built on ETFs," said Polfrone.

Many providers to the 401(k) market expected Schwab's ETF 401(k) offering, launched at the beginning of 2014, to push awareness, demand, and ultimately ETF product offerings to plan sponsors.

But adoption has been slow. Schwab offers its ETF option to plans with a minimum of $20 million in assets. For larger plan sponsors with access to institutional shares of indexed mutual funds, the value add of ETFs is watered down, say critics of the products in defined contribution plans, as expense ratios of institutional class shares of mutual funds are often even more competitive than ETFs.

A 2013 Fidelity report said "index mutual funds may be more suitable than index ETFs for workplace savings plan participants seeking passive exposure," arguing the mutual funds offer the same "key advantages as ETFs" without the potential to incur the expensive trading costs associated with ETFs.

Nonetheless, Schwab says its data shows that its all-ETF 401(k) platform can cut plan costs by up to 30 percent compared to plans build on indexed mutual funds.

Other indications suggest ETFs might be ready to claim more attention from plan sponsors.

This week, ShareBuilder 401(k), a provider of all ETF 401(k) plans the company says can benefit companies small and large, announced a promotion that it was waiving plan implementation fees though May 17.

And TD Ameritrade, which reported $16 billion in new retirement assets last quarter, has been marketing the ETF option through its TD Ameritrade Retirement plan, rolled out in 2014 to its channel of 4,500 RIAs.