Lack of shirt sponsorship in US sports.

[Bored Of Education]

I have to say that speedway is incredible for sponsorship

[Sam]

Also, of course, NBA jerseys, in themselves have less material to place sponsors' logos on.

I'm not sure that's the case. I mean sure, they don't have sleeves. But that's cancelled out by the fact that they're being worn by some seriously large human beings. Combined with the current slim fit of football shirts, I reckon the amount of material used for the shirt of any footballer smaller than Harry Maguire (who I first noticed during the World Cup as the bloke who looked like someone had stretched a football shirt over a refrigerator) is going to be less than that used for a fairly thick-shouldered vest worn by someone who's pushing seven feet tall and is built with it. And some of them even wear t-shirts rather than vests these days, you know.

[Gerontophile]

It only started in the NBA last season.

Though it is worth noting that WNBA teams (many of which are co-owned with NBA teams) have had full-fledged jersey sponsors for five plus seasons.

There's a reason why nobody noticed.*

And the Las Vegas Lights (2nd tier men's soccer) had a sponsorship deal before they had a name.

*Sorry, that sounded a little more sexist/misogynist than I meant. (I only meant to be a little...)
The crowds for the WNBA in Las Vegas are huge. But it's mostly freebies/comps/reduced prices. And they ain't winning.