Just over a month ago, the ARK Innovation ETF (ARKK) took home ETF.com's "ETF Of The Year" award for delivering on its purpose of serving up access to disruptive technology in 2017, and doing so really well.

Crucial to that recognition was its unique allocation to bitcoin. For much of the fund's history, that allocation was relatively stable in number of shares, and hovered between 6 percent and 10 percent of the portfolio, according to FactSet data. Bitcoin often led the list of top holdings in ARKK. Only one other ETF on the market offered parallel access to bitcoin — ARKK's internet-focused counterpart ARK Web x.0 ETF (ARKW).

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Bitcoin, accessed through the Bitcoin Investment Trust (GBTC), was a major driver behind both these funds' returns last year. ARKK and ARKW were each up more than 87 percent in 2017. GBTC itself was up a whopping 1,550 percent.

Here's the ETFs' rally last year, and that impressive rally relative to GBTC's, in two charts:

But these ETFs' big bet on bitcoin is no longer the case. In mid January of this year, ARK began trimming exposure to bitcoin in both funds, and today bitcoin represents only 0.5 percent and 0.6 percent in ARKK and ARKW, respectively. The cryptocurrency is almost a negligible allocation in these portfolios now.

Why? According to ARK, the "complicated decision" to dramatically trim bitcoin exposure was more driven by regulatory and tax-related concerns than by the "merits" of bitcoin itself.

But performance probably played a major role in this portfolio shift. Bitcoin — as measured by GBTC — rose dramatically in 2016 and in early 2017, and then began to fall later in the year. So far in 2018, GBTC has already dropped 37 percent, bringing its losses from its mid-December 2017 high to more than 63 percent in just five months. (ARKK and ARKW are each up about 15 percent year-to-date.)

It could very well be that ARK was simply taking profits on its bitcoin allocation earlier this year, and has been trimming that position since as bitcoin prices continue to struggle. If GBTC price performance did in fact motivate the move, any recovery in bitcoin prices could mean an increase in allocation down the road.

For now, however, given that the Securities and Exchange Commission has yet to approve the launch of a bitcoin ETF — many issuers have thrown their hats into that ring — investors who want access to the cryptocurrency through an ETF wrapper have slim pickings at best.

— By Cinthia Murphy, ETF.com. Contact Cinthia Murphy at cmurphy@etf.com