Just curious as to whether there was an interesting reason.
For example, MLB's (US Professional Baseball) looks like this, NBA's looks like this.
Meanwhile, the rest look very different: NFL, MLS, NHL.
While I know some context exists, as the NHL for example is two countries and the MLS is very new, I guess I'm very curious as to why the NFL didn't adapt the same style as MLB and the NBA.
This is probably a better question for a graphics person, as the history really isn't that interesting. I'm sure the first thing you'll note is the colors used. MLB, NBA, and NFL all have the "patriotic" red, white, and blue.
There are two main styles for the logos (I prefer to discuss the Big 4 sports, but we can talk about MLS too): Shield and Silhouette.
The shield style logo is the traditional one that is meant to invoke the old heraldic emblems. As for why the NFL didn't adapt the same style as the MLB and NBA, the shield style is the older of the bunch. The NFL has been using a shield style logo since the 1940s, with a few minimal redesigns throughout the years. The first NHL logo was actually orange and black, existing since the beginning of the league in 1917, and stayed the same for over 75 years. The new "retooled" NHL logo changed the color scheme to silver and black in an effort to model itself after the Stanley Cup and give it that sleek style that many organizations strove for in the 2000s.
The MLB and NBA logos are the newcomers of the bunch and have adopted the silhouette style logo. The MLB silhouette is quite notable for the ambiguity of it. One semi-popular legend is that it was based off the silhouette of Harmon Killebrew, but that's been debunked numerous times (though it keeps cropping up). The logo is intentionally ambiguous. You cannot tell whether the batter is a righty, or a lefty, or anything about the racial characteristics. They adopted that logo in 1969 and have used it pretty much unadultered since. The NBA logo came in around 1970 and features the silhouette of basketball great Jerry West. It's interesting that the logo has stayed the same, especially given the fact that the ABA and NBA didn't complete their merger until 1976. Even though the NBA had the greater bargaining position, you wouldn't be amiss to think that they would redesign the logo as some sign of reorganization/reconciliation/etc. But the iconic Jerry West has been the logo ever since 1970.
MLS is the newest of the bunch and they're trying to do something similar to the MLB where the shoe can stand in for any player, regardless of race, etc. I really can't speak much for MLS since I don't know much about it. But if we're going to mention MLS, you may as well mention the WNBA (started with a shield style silhouette, but retooled it to a new silhouette in 2013), or the current AHL logo (a circular silhouette).
Again, there's little to be said about the history of WHY they look as they do, outside of the heraldic style of the earlier period and the all-inclusive ambiguous silhouette of the later period. You'd want an art historian who does 20th Century graphic design to answer this, but I don't know of any really.
