Question: In the US, there are many large cities that are dominated by a single party. After Reconstruction and through the 70s, the Southern States were solidly Democratic.

There are often strong factions in these single party districts, but they do not become full parties with clear signalling mechanisms and separate governance structure. 

Why not? What are the theories of why local parties do not emerge?

Are there any examples of local parties emerging in the US?

Answer: 
Broadly speaking: demographics. In the US, rural tends to lean republican and urban leans democrat. We also have bigger regional leanings (like New England vs. Texas). On top of that, we have gerrymandering where a party that is in power draws the district lines in hopes they can stay in power. 

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> What are the theories of why local parties do not emerge?
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They do emerge, but are often at a disadvantage against candidates from the major parties, who find plenty of support being a part of the bigger machine. 

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> Are there any examples of local parties emerging in the US?
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Yes. Some that come to mind would be:

* [The Independence Party of MN](https://en.wikipedia.org/wiki/Independence_Party_of_Minnesota) who famously got [Jesse Ventura](https://en.wikipedia.org/wiki/Jesse_Ventura) elected as governor.
* [Socialist Alternative](http://www.socialistalternative.org/) in Seattle who got [Kshama Sawant](https://en.wikipedia.org/wiki/Kshama_Sawant#Seattle_City_Council) elected to city council.
* Wikipedia also has a [long list of regional parties in the US](https://en.wikipedia.org/wiki/List_of_political_parties_in_the_United_States#Regional_parties).
My answer is a bit more narrow than the others. Third parties in the US are rarely successful due to the first-past-the-post voting system, otherwise known as winner-take-all voting.

Imagine that a party that mostly aligns with the democrats but has some slightly different views rises up in one area that has a history of voting about 60% democrats 40% republicans in most elections. When it comes to election time they've managed to sway about half of the democrat voters and the election results come out like this:

* 30% democrats
* 30% third party
* 40% republicans

The republicans win even though 60% of voters don't want them in government, at the next election, everyone gives up on the third party and goes back to voting democrat because they have the best chance of winning, the third party all but disappears and everything basically goes back to normal.

This is known as vote splitting and leads to factional divisions and power struggles within a party in favour of forming new parties, it also means that it normally takes a massive effort to upset the status quo of a political system that uses this voting system.

If you want to learn about alternative systems of voting I highly recommend watching CGP Grey's series ["Politics in the Animal Kingdom"](http://www.cgpgrey.com/politics-in-the-animal-kingdom/) and researching Condorcet voting.