They're not as isolated in that regard. there's a number of other churches and cathedrals in the immediate area, some of the most famous being :

• San Vitale in Ravenna
• Cathedral in Parma
• Euphrasian Basilica in Porec

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Not to mention a detached campanile is a stark element in a number of other Italian churches (San Marco's in Venice being the most famous one)

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As to how and why : All of the said churches were a) formed in the pre-Romanesque and Romanesque period b) were either large cathedrals i.e. seats of bishops, or churches in important cities.

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In early christian architecture, two important things shape these idiosyncracies :

• Given the 'collapse' of the political power after the fall of the Roman Empire, and the rise of a new religion, we're left with a number of old judicial buildings that no longer have a purpose (basilicas) and the new religion requires new places of worship, a larger one than the ancient pagan temples, to house the congregation of church during its rituals. Ergo, basilicas are soon repurposed into churches, and even now the term 'basilica' invokes the image of a church, not of a judicial building as it originally was.
• Given the number of converts to Christianity, which are mostly adults, the baptisterium can't be a small pool of water in which children are baptized today in Christian rituals; a smaller church somehow manages by integrating a larger baptistry within the church, usually in the transept or the main nave, a larger and more important one builds a detached baptisterium building.

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So we have these preromanesque elements - a building with the foundation set in the old era, which functions as a gathering hall, but suddenly requires two new elements - a belltower that calls the congregation to mass, and a baptistry, large enough to allow adult converts to be baptized in it. In important, large places, these are given their separate detached locations, owing to a number of factors - the fear to build onto p of existing buildings, the availability of space, the prestige of making a 'new' building. Eventually, the building of detached campanile and baptisterium became a style idiosyncracy of Italian Romanesque, the Romanesque style itself being very eclectical.

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After the Romanesque period, these huge baptisteriums weren't required and have fallen out of fashion, most of them being torn down after they outlived their purposes as the church building was radically reconstructed in the new, Gothic style. A good example of this is the Reims Cathedral, the traditional place of crowning of French kings. This all started when Clovis I. converted there to Christianity in 508. and was baptized in a detached baptisterium. We know this because its foundations were excavated in front of the cathedral's entrance, and my point is, even an important detached baptisterium like that didn't survive falling out of fashion.

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The churches that kept their baptisteriums and campanile as detached buildings have never went through a gothic reconstruction, staying in their original state. ( Santa Maria del Fiore in Firenze went through gothic touch-ups on the campanile and baptisterium by Giotto, and the new dome by Brunelleschi, but these projects were separate and weren't intended or done as radical reconstructions and the romanesque base was untouched )