By Brad Tuttle, MONEY

When Toyota released its December 2014 results this week, the automaker highlighted how—like most of the industry—sales have been booming. Toyota (TM) sales in the U.S. jumped 12.7% compared with the previous December, and they were up 6.2% for the year as a whole. The announcement also played up the fact that Lexus had its best sales month ever in December; that sales for trucks, SUVs, and the Sienna minivan were all soaring; and that the Camry held bragging rights as America’s best-selling car, a title it’s owned for 13 years running.

What’s just as interesting about the announcement is the car model that’s notably absent: Toyota Prius. The world’s best-selling and best-known hybrid vehicle, the pioneering Prius, is not mentioned in one Toyota 2014 sales press release and is downplayed in another, with only a quick line stating “we sold more than 200,000 Prius for the third consecutive year.”

Understandably, Toyota is trying to accentuate the positive in 2014 sales, so let’s turn to the auto resource site WardsAuto, which states explicitly that the Prius’s 207,372 units sold represents a 11.5% decrease from 2013. USA Today recently called on another sales data source to report that through the first 11 months of 2014, Prius sales were down nearly 16% compared with the prior year. What’s more, according to the Detroit Free Press, overall sales of gas-electric hybrids like the Prius were on pace to fall 9% for the year.

In 2013, gas-electric hybrids accounted for 3.2% of all light vehicle sales in the U.S. Last year, that figure dipped to just 2.8%. This isn’t remotely the trajectory most experts anticipated. A J.D. Power forecast made in 2008, when hybrids were 2.2% of U.S. car sales, predicted that the category would constitute 7% of the market by 2015.

What happened? The short answer is: cheap gas prices. Oil prices have plunged since summer and have just kept on falling. The consensus says that the result will be inexpensive prices at the pump for the indefinite future. According to AAA, the national average for a gallon of regular was $2.20 as of Monday, roughly $1.10 cheaper than one year ago.

The plummeting price of gasoline has surely played a big role in hot sales for SUVs and luxury cars on the one hand, and the struggles of the Prius and hybrids on the other. In December, a Businessweek article argued that with $2 gas being commonplace, the Prius is only viewed as a smart financial choice by drivers “who stink at math.” Researchers factored in the upfront costs of the Prius and a similarly equipped gas-powered Chevy Cruze, and then did the math on how long it would take for the pricier Prius to pay off via savings on fuel. The answer was that with $2 gas prices, you’d have to own the Prius for 28 years to break even compared with the overall costs of the Cruze.

But cheap gas is only part of the reason why Prius sales are on the decline. Karl Brauer, senior director of insights for Kelley Blue Book, explained that “the Prius had a good thing going for several years as the ‘official’ vehicle of the environmentally conscious,” a reputation that was solidified during the 2003 Academy Awards, when dozens of celebrities arrived in chauffeur-driven Priuses. The cachet of the Prius has dissipated in the years since because, among other reasons, its fuel efficiency advantage over the competition has shrunk substantially, and Tesla has emerged as the green car of choice that’s not only environmentally friendly, but stylish and a rip-roaring hoot to drive as well.

“A lot of vehicles today get 40+ miles per gallon and you don’t have to make the sacrifices you do with the Prius,” said Brauer, pointing to the fun-to-drive Volkswagen Golf and the surprisingly spacious and practical Honda Fit as appealing, fuel-efficient alternatives to the Prius. “And the Tesla has hurt the Prius as much as anything else.”

What’s especially interesting is that while low gas prices appear to be a factor in declining sales of the Prius and other hybrids, cheap fuel doesn’t seem to be cutting into sales of some purely electric-powered cars, like the Tesla Model S and the Nissan Leaf.

A recent study conducted on the behalf of NACS, a convenience store and retail fuel association, estimates that each 10¢ drop in gas prices correlates to a 1% decrease in consumers who would consider alternative-fuel vehicles. As of November, for instance, 34% of Americans polled said they would be interested in an all-electric vehicle such as the Nissan Leaf, compared with 55% in April, when gas prices were roughly 90¢ per gallon more expensive.

And yet, curiously, while sales of the Prius and other hybrids have suffered hand in hand with falling gas prices, the Nissan Leaf has had a record year. Nissan sold 3,102 Leafs in December and 30,200 Leafs for all of 2014, up from 2,529 and 22,610, respectively, the year before. Likewise, even though the Model S wasn’t new in 2014 and had a high starting price of around $70,000, Tesla sold about as many of the models last year as it did in 2013 when it was the absolute darling of the industry.

One explanation for why sales of pure electric vehicles haven’t slumped like hybrids is that in certain circles EVs are viewed as superior in terms of environmental friendliness and just plain coolness. Then again, it must be pointed out that even as Prius sales decline, it still outsells the Nissan Leaf by a factor of nearly seven.

A 2016 model year Prius is expected to hit the market this year, and Brauer said that, in light of EVs holding the edge in terms of being the green choice, and vastly improved fuel-efficient mainstream vehicles being the smarter economic option in an era of cheap gas, Toyota faces real challenges getting sales back on the upswing. “They have to make the Prius appealing beyond the green car claims,” he said. “The ‘green’ has to be gravy on top of what’s a fun car to own and drive.”