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Light beer used to be funny, with ads featuring talking chimps and men who wear dresses to score discounts at the bar on ladies’ night.

But light-beer sales have fallen off a cliff since 2007 and become such a problem that the decline is a significant factor driving a merger boom in the beer industry, including the prospect of a deal between the world’s two biggest brewers: Anheuser-Busch InBev NV and SABMiller PLC.

In September alone, three big brewers announced craft-beer acquisitions aimed at fueling growth or offsetting some of the big-brand malaise. Those include Heineken NV, which took a 50% stake in Lagunitas Brewing Co. for an estimated value of $400 million. On Sept. 16, AB InBev, the world’s largest brewer, said it approached SABMiller about combining the two companies. A deal would reduce AB InBev’s dependence on the U.S. market at the same time that sales of Bud Light have declined for six straight quarters. It has until Oct. 14 to make an offer.

In the U.S., the world’s most profitable beer market, the top U.S. brands had a dismal summer even though it is their peak season. Volumes of AB InBev’s Bud Light declined 3.3% while those of MillerCoors LLC’s Coors Light fell 0.7% for the three months ended Sept. 5, according to Nielsen data cited by Morgan Stanley. Only Miller Lite bucked the trend, rising 1.8% during the period—a positive for a brand that has reported volume declines for seven straight years.

Until this summer, lower gasoline prices—a boon for discretionary spending—were expected to boost light-beer volumes, which are off 12% since the major brands’ market share peaked in 2007.

Now the brewers are firing their advertising agencies and changing packaging. MillerCoors, the second-largest brewer by volume, ousted two top executives and said it will close a brewery.

The industry’s light-beer problem was on full display at Buckhead Saloon in Atlanta on a recent Thursday night. Dani Fleck, a 24-year-old occupational therapist, and Casey Jones, a 24-year-old financial adviser, sipped on Hi-5 IPAs from Terrapin Beer Co. of Athens, Ga., before switching to Bud Light, which was being given away as part of a promotion.

Mr. Jones said he drinks craft beer with dinner or on nights he plans to have only a beer or two, and Bud Light on weekends at the lake or during college-football tailgate gatherings, when he plans to drink more, because it is easier to drink multiple light beers than a heavier, more alcoholic craft brew. He added, “Craft beers are taking over.”

Drinking behaviors like these have cut the top light-beer brands’ share of the $100 billion beer market to 31.8% from a peak of 35.5% in 2007, according to industry tracker Beer Marketer’s Insights. Craft beer’s market share doubled to 9% over that period, and imported beer rose slightly to 14.1%.

This has created a sense of urgency at AB InBev. In July, Bud Light fired BBDO Worldwide Inc. and hired Wieden+Kennedy, its fifth advertising agency in five years. Anheuser-Busch Marketing Vice President Jorn Socquet said BBDO wasn’t doing “breakthrough work.” A recent ad in the brand’s “Up for Whatever” campaign showed a man in his early 20s playing a life-size game of Pac Man. Mr. Socquet said the campaign was hedonistic and “focused on live for today, not for tomorrow.”

BBDO declined to comment.

MillerCoors ousted its chief marketing officer, Andy England, in July. His successor, David Kroll, later cut ties with Coors Light’s ad agency, Chicago-based Cavalry, and hired 72andSunny. The changes came less than a year after the brewing company’s Miller Lite brand hired TBWA Worldwide Inc., its third ad agency in three years, and returned to its original white packaging and blue logo. The packaging has helped Miller Lite increase volume in recent quarters.

Light beer was created by Miller Brewing Co. in the 1970s, when sales of low-calorie products such as diet soda and margarine were taking off. Miller Lite quickly became the nation’s second-largest beer by volume behind Budweiser, because the brand persuaded men that “diet” beer wasn’t for sissies with ads featuring former professional athletes and other celebrities touting its “Tastes Great, Less Filling” slogan.

Miller Lite now ranks as the No. 4 beer, after Bud Light, Coors Light and Budweiser.

Light-beer ads eventually became silly. Magical trees grew Bud Light bottles and comedian Rodney Dangerfield hit trick golf shots. The humorous commercials drove sales because people associated beer with having fun, said Bob Lachky, Anheuser Busch’s former chief creative officer.

But the silliness gave way to sophomoric humor: women mud wrestlers, talking dogs (saying “sausages, sausages, sausages”) and a fighting squirrel. Cavalry Chief Executive Marty Stock said the quality of humor deteriorated over the years because of executive turnover and changes in ownership at MillerCoors and AB InBev around that time. The sophomoric ads made light beer a punch line in the craft-beer age.

Mr. Socquet wants to change the negative perception craft brewers have stoked that “we’re just a light beer.” Spots next year from Wieden will play up Bud Light’s quality—AB InBev says it takes twice as long to brew as many other beers—and its role in bringing people together. For example, a current ad shows a group of 20-something friends sampling Bud Light from tasting glasses. AB InBev also introduced new packaging featuring the logos of National Football League teams.

MillerCoors plans to focus new marketing on women. Company research shows they account for 25% of light-beer consumption, and Mr. Kroll said they represent a “disproportionate share” of light beer’s decline because they are drinking more cider, wine and liquor. He added that spots are more likely to be “clever and witty” than sophomoric.

“Getting American light lager back to growth is going to be a journey,” Mr. Kroll said.