Is there a specific area you're thinking of? Do you have what we refer to as "County Islands" in your area that are small dots of unincorporated land inside the City?
The first thing is unincorporated areas don't form, every area was originally unincorporated and the City forms. All land is 'unincorporated' until a City is created that changes its municipal structure. So a few ways this happens are:
1) that area was originally excluded possibly because it was some sort of County govt parcel or other specific feature they wanted kept out of the City.
2) cities expand over time by annexing parcels in. Typically there are City initiated annexation and owner annexations. If someone was opposing a City action and had political support or was just too ornery they may have been left out on purpose.
3) some areas look like they make sense for a city service, but there may be some underlying issue. Locally I know of one that water/sewer makes more sense from the adjacent City than the city its supposed to end up in. So we're coordinating with both cities to essentially be approved by the current city and then 'transfered' into the city that will actually provide service.
Every state has their own rules about incorporating or chartering a new city or municipality. This answer will be brief and not all inclusive.
To become a municipality the local residents must band together and choose to create one following state guidelines. They must establish boundaries and elect officials. The officials can be paid or volunteers.
Generally speaking unincorporated areas are sparsely populated and cannot meet the state guidelines for incorporation in regards to either population density, or they are located too close to an area that has already incorporated.
There will always be exceptions. My father and I used to go to Jadwin, MO to rent canoes for float trips. Jadwin Canoe rental was owned by the Jadwin family. The Jadwin family home was pretty much the entire town back in the 70s.
In short unincorporated just means that nobody ever bothered to incorporated the location due to the cost and hassle. So they never formed they are unformed.