But why, exactly, is that the case? Here are a few theories:
1. Smaller teams, closer fans and wearing warmups: Many of the protests above involved warmups. The NFL and Major League Baseball don't wear warmups before games, and even though the WNBA does, it doesn't have nearly as wide an audience. If NBA players want to make a statement with their clothing, it's easy to do before a game. They don't have any equipment to hide whatever message they want to make, and with so few of them on the court, they're hard to miss. And it doesn't hurt that they are literally a few feet from their audience and the media covering the event.
2. Their commissioner is pretty lax on fining players for protesting: When players began wearing "I can't breathe" shirts, NBA commissioner Adam Silver issued a statement saying he respected players for "voicing their personal views," but preferred they "abide by [the league's] on-court attire rules." It was a weak request -- a result of trying to enforce NBA dress rules while being sensitive to the situation, and no players were fined. (For what it's worth, the NFL also did not fine St. Louis Rams players for making the "hands up, don't shoot" gesture at a December game. This, though, didn't involve breaking any dress codes.)
3. The issues they protest about are political, but not partisan: With the exception of Phoenix protesting Arizona's SB 1070, the players aren't protesting specific legislation or candidates. They're being political, but not necessarily partisan (although depending on the specific situation you're asking about, opinion can break down on more partisan lines). If you had, say, players coming out wearing campaign buttons the week before an election, it might be another matter.
4. The league has more black players -- and fans: According to the Institute for Diversity and Ethics in Sport, 77 percent of NBA players are black -- a higher percentage than the WNBA (69 percent), NFL (67 percent) and Major League Baseball (8.2 percent). NBA fans are also 45 percent black, the highest of professional sports and three times higher than the NFL or NCAA, according to the same study. And many of these players grew up in the same neighborhoods; both Anthony and Gray grew up in the west side of Baltimore, for example. Basketball thrives as a sport in the inner cities in a way that baseball and football simply don't. So for more athletes (and fans), some of these issues are personal.