Ford has a bunch of new TV ads for its all-new F-150 pickup, but they won't be running during the Super Bowl. Source: Ford Motor Company.

For many Americans, watching the Super Bowl isn't about the football -- it's about the ads.

Fans of the Seahawks and Patriots may be focused on the game this year, but for everyone else who tunes in, the commercials could provide a big part of the evening's entertainment.

But this year's batch of Super Bowl commercials will be without some of the biggest names of years past: Both Ford (NYSE:F) and General Motors (NYSE:GM) say they'll be sitting on the sidelines this year.

No Ford truck ads this time around...
With this year's rates hitting a reported $4.5 million for a 30-second slot, ad time during the Super Bowl is among the world's most expensive. But with a huge audience that is primed to focus on the ads, it can be among the most effective -- for the right brands, with the right commercials.

However, both Ford and GM say they're forgoing the opportunity this year. Ford confirmed earlier this week that neither the Ford nor Lincoln brands would be running commercials during Super Bowl XLIX. 

Ford's decision might surprise regular football-watchers: The Blue Oval's truck commercials have been ubiquitous during NFL broadcasts for a few years now. But Ford hasn't been a big player in the Super Bowl (so to speak) in recent years. The company ran an ad for its Fusion Hybrid before last year's game, and Lincoln ran some ads during 2013's edition, but Ford hasn't made a big bet on the big game in several years.

...and no Chevy ads, either
Likewise, it's something of a surprise for General Motors to sit out as well. The General won't be completely absent from the event itself -- the game's Most Valuable Player will be presented with a new Chevy Colorado pickup -- but it's not buying any ad time during the television broadcast.

GM drew surprised reactions when it skipped the 2013 game after then-marketing-chief Joel Ewanick said the company couldn't justify the expense of the ads. It returned last year, with two 30-second spots for the Chevrolet brand -- but like Ford, it decided to pass this time around.

Why? Executives at both automakers have hinted that the timing doesn't work for them this year. Automakers like to make big advertising pushes to coincide with new-product launches, and neither has a significant product launching right now. But it's also true that the cost of Super Bowl advertising has risen to the point where an established brand with no big news to announce might decide that it fails the cost-effectiveness test.

Some automakers have jumped in, but not as many as in the past
GM and Ford aren't the only automakers sitting out this year's tilt. Audi, Honda, and Jaguar have all had Super Bowl ads in recent years, but all will be absent from the 2015 edition.

In fact, auto ads have been a tough sell for NBC Universal this year, according to a report in Advertising Age. Last year, 11 different auto brands bought ads, accounting for over a quarter of the game's total advertising -- but this year, only six brands are expected to run ads.

Those brands include luxury stalwarts BMW and Mercedes-Benz, Toyota and its Lexus brand, Kia Motors, and Nissan

Super Bowl ads can generate massive consumer interest, even in the premium segments. While Audi is sitting out this year, it has had great success with its ads in the past, going back to 2008. Fiat Chrysler wowed audiences with its "Imported from Detroit" ads a few years ago, and saw its sales and consideration spike sharply in the months that followed.

Last year, its Maserati brand ran just one spot last year (and a somewhat mystifying one at that), but it generated a surge of interest in its then-new Ghibli sedan -- kicking off a year in which it saw U.S. sales rise 110%.

The upshot: Other priorities take precedence for many
GM's Cadillac brand is taking on BMW and Mercedes in a big way -- but it's planning on making its big TV-ad move during the Oscars, next month. GM officials think the Academy Awards telecast might offer a richer audience for Cadillac -- and the ad time will surely be less expensive.

Likewise, Ford is spending big to promote its all-new F-150 pickup, but in ways that are more directly targeted to its intended audience. Toyota, BMW, and a few others still find the big game to be a worthwhile place to advertise -- but Detroit's big guns are playing elsewhere this year.