The numbers here come from the Sierra Club, which analyzed advertising trends in the US, and found that the number of ads for non-plug in vehicles far, far outnumbered the ads for the more future-focused cars. For example, Ford was the source of that 4,750 number, with that many national TV ads for the gas-powered Focus, but it only advertised the Focus Electric about 200 times. Mercedes put up national TV ads for the C-Class 1,400 times, but couldn't be bothered to try to sell its B-Class electric vehicle on TV in any way. Probably because Mercedes execs aren't yet convinced that TV ads work.
The companies that are actually in favor of EVs (according to the Sierra Club's numbers, that'd be Nissan and GM), the ad buys are (surprise) not nearly so lopsided. Nissan put out 1,750 nationwide TV ads for the all-electric Leaf, compared to 3,500 for the somewhat comparable Sentra. GM promoted the Cruze 700 times on national TV, and the Volt 200 times. You can find all of the details over on the Sierra Club website, along with pretty charts.
We will say that there are more models of gas-powered cars than plug-ins, but, of course, there's nothing stopping Ford, for example, from building a PHEV version of the F-150. If the company wanted to, it would. Wasn't it someone in the company history who once said, "If I'd asked my customers what they wanted, they'd have said a faster horse"? Okay, maybe the quote itself is apocryphal, but the tone certainly seems accurate. Today, too, the images that the automakers beam into our brains are powerful ones, and if they're not going to show us some EVs, other methods might be necessary.
But, you might be saying, most of the EVs are sold in California, so it makes sense to advertise there. Well, you're aware of the chicken and egg conundrum, right? Yes, California offers state incentives on top of the federal ones, but it's not the only state to do so. And there are no rules against EV sales outside of special incentive zones.