American teams are always named as [city/region team is from] + [team's actual name]
Los Angeles Lakers, Denver Broncos, NY Yankees, Chicago Fire, etc.
Is there a specific reason for that?
The answer to why all the U.S. professional teams are named this way (Location Nickname) today is because it is tradition. Baseball teams were named this way, and football, basketball, hockey, and soccer (in the U.S.) all came after baseball.
Baseball teams weren't always named this way, however. The National Association of Professional Base Ball Players was the first professional baseball league (1871-1875). A few teams in this league had modern sounding names (Boston Red Stockings, Chicago White Stockings), but many teams in this league had official names like Mutual Baseball Club of New York, Athletic Base Ball Club of Philadelphia, Union Base Ball Club Lansingburgh, Lord Baltimore Club, etc. Box scores in the newspaper would shorten these club names to one word, based on either their city or their club name: Boston, Chicago, Mutual, Athletic, Troy, Lord Baltimore, etc. The sportswriters would give the teams their own nicknames, usually based on the colors of their uniforms or the name of their field. For example, the Union Base Ball Club Lansingburgh played at Haymakers' Grounds in Troy, NY. As a result, their team was often called the Haymakers or the Trojans. Lord Baltimore got the nicknames Canaries and Yellow Stockings, because of the uniforms. Modern sportswriters, in writing about the history of these old teams, have retroactively assigned modern style names to some of these teams: they are now referred to with names New York Mutuals, Philadelphia Athletics, Troy Haymakers, and Baltimore Canaries.
In 1876, six clubs from the National Association broke off and started the current National League, including two teams with old style names: Mutual and Athletic. Those two teams lasted only one season, and all that was left in this baseball league were teams with the modern name format.
There are several advantages of the Location Nickname format. When reading or hearing about the team, you immediately know where the team is located. It allows for there to be more than one team in a single city or more than one team with the same nickname, yet have unique names for each. It also is an attempt to build a fanbase; having your city name (or region name) in your team name is supposed to signify that you are the official team of your city, and everyone who lives there should be a fan.
In most places in the world there is a considerable amount of space between different competing teams in the same league. The naming convention used in the US is not just the US but is also many/most sports teams in the world. I played a little basketball in France and yes all of the teams use that convention, albeit with more emphasis on the city name.
Really it is either about league rules or selling tickets or both. You want to associate with a local city with a high population because most of the tickets will need to come from that fan base. I would say that many teams in the big 4 sports in the US have stadiums that are not in the city of their name but often a town close to the bigger city.
A couple of examples:
Why Manchester United and other teams in the UK don't follow that trend is exactly the same reason. There are a lot of teams competing for fans that can all drive to their games. Also the actual cities where they play aren't that big comparatively to the US based metro areas. So the last thing they want to do is put a city name in front and then possibly disengage fans from that team because they don't live in that city or don't like that city.
There are leagues that have naming rules but really I doubt a league would care unless there was vulgarity or some insensitivity in the name. Really the league wants teams to be successful and has to respect a team's decision to name their team what they want to help sell tickets and stay afloat.
Note: In relation to Ben's answer, which is good, I would like to add that in many parts of Europe (I honestly don't know about Asia/Africa so chime in if you do) sports were regulated through local clubs.
In the US for the most part sports are denoted by town when growing up (you play for Smithsville little league), then by school (high school and college) then by town/mascot for professional.
This is very different from the club system which we have in the US (soccer and AAU basketball to name to biggies) but they are no where near as prominent as Europe where for the most part schools have nothing to do with sports and everything filters through club systems.