I can't speak to NBA, but I'll try to give a little context for the NFL one.

First and probably foremost, Jackie Robinson ended up having an incredible career. Even with a late start to MLB he played for 10 years and was still a decent player in his twilight years (he hit over 300 every year between ages 30-35!). He also had a large charismatic personality.

Plus, at the time baseball was THE national sport, especially compared to professional football which didn't start to get national popularity until later on. Here is some information from WSJ comparing the two popularity, although only the top chart goes back that far.

[While there were a handful of black professional football players] (https://en.wikipedia.org/wiki/Black_players_in_American_professional_football), the first NFL player was Kenny Washington in 1946. Basically, the Rams were pressured into it as part of them moving to LA. They signed the hometown hero and UCLA legend Washington. Unfortunately, Kenny's career was already on the downswing by this point and his knees were shot. He had a productive but short-lived three year career before retiring. He never got the chance to make waves like Jackie Robinson and Larry Doby did (the second black MLB player).

Editing in a fun fact: Rams are not only the first team to sign a black player into the NFL, but they were also the first one to sign an openly gay player, Michael Sam

Though not included in the question, I can provide some discussion about the first black player in NHL history, Willie O'Ree, and why he is not widely discussed.

As the question kind of implies, there are not a lot of black players in hockey. As this somewhat outdated Wikipedia article shows their numbers are limited; roughly 20 players have played in the NHL this year (out of about 1000 in total to appear in one game), and the costs associated with hockey mean it is largely an upper-middle class and (majority) Canadian sport, categories that is predominantly white. So just from that the impact of a black player is not as major as it is in the other three leagues in North America, where blacks have far larger representation. Thus it isn't surprising that O'Ree's impact is a lot less known or discussed than someone like Robinson. But it still matters, and is important.

Baseball is known for its unofficial policy of segregation, which as noted ended when Robinson began playing in 1947. In comparison, the NHL did not, but that was more to the lack of black hockey players (note: there were black players, and a prominent black league based in Nova Scotia at the start of the 1900s; but again, the numbers of players was even more drastic than today). It was also wary of issues that may have occurred, even in Canada. There is a legend that Conn Smythe, the owner/manager of the Toronto Maple Leafs, once said in 1938 that he'd offer $10,000 to anyone who could turn Herb Carnegie white. For context, Carnegie was a well-known black player in the Quebec senior league (a step below the NHL at the time) and was at one point even offered a contract by the New York Rangers (he refused due to the low salary; whether that low salary was racially-motivated is not known, considering hockey players were indeed underpaid at the time). But that the possibility of Smythe saying that shows that there was reluctance to sign black players at the time.

Now onto Willie O'Ree. Like Carnegie he played in the Quebec senior league, and is also notable because he is 95% blind in one eye (which should have forced him from the NHL, but he cheated on the exam). In 1958 he was an injury call-up, and played two games for the Boston Bruins. He would not play again in the NHL until 1960-61, when he appeared in 43 more games, but spent the rest of his career playing for California-based teams in the minor professional Western Hockey League, retiring in 1974. All told, O'Ree was in 45 NHL games in his career, and had 4 goals and 10 assists for 14 points.

O'Ree was not that noteworthy for a long time, in part because he didn't have an impact like Robinson did, as in he was frankly not that great in the NHL. As well it would not be until 1974 that the second black player, Mike Marson, appeared in the NHL, and only in 1981 that the first real "star" player, goaltender Grant Fuhr, made his debut.

But in the past decade or so, the NHL has really stepped up its promotion of O'Ree and his role. He has served as their public face for programs that encourage black youth to play hockey, which has seen moderate results. But simply put, because the lack of black hockey players, and the relative obscurity of O'Ree as an NHL player, his story is not widely known.

That said, there is a fairly good book on the history of black hockey players: Breaking the Ice: The Black Experience in Professional Hockey by Cecil Harris (2003) chronicles everything up until 2002, when Jarome Iginla (who's father is Nigerian) broke out and nearly won the league's MVP award, while leading the league in goals and points. It's no an academic work by any means, and is certainly biased, but it covers a subject that is really underwritten about.