Before I can answer why there are so many different pagan pantheons in existence today, I first need to clarify what pagan and pantheon mean. The word "pagan" is a simple word meaning country dwellers. This title was given, by the church, to native peoples who continued to followed old traditional religions in the early days of Christianity. Later, this term was used as a catch-all umbrella for any and all people who chose and choose to follow any religious belief structure that is not a part of the widely accepted Judea Christian sects. The word pantheon describes the set lists of Gods or even Goddesses that any particular Pagan group or individual choose to follow. Now that we know what those two words mean, there are three main reasons why there are so many different pagan pantheons in existence today.
The first has to do with global diversity. Christianity is a globally accepted religion today, but in the years prior to 2011 B.C. each region around the world chose their own deity or deities to worship and follow according to their legends, experiences, and ideals about creation and the afterlife. For instance, older farming communities worshipped nature based deities that they believed controlled the harvest and crops, and the ancient Greeks believed that there were a great number of both Gods and Goddesses who each reigned over a particular aspect of life.
For the Greeks, they believed in a hierarchal society where everyone had a place and there was a place for everything, and so too was their pantheon. Many years later the Romans came along and adopted the Greeks pantheon but in doing so changed all the names of the original Gods to fit with their own society. By doing this a second pantheon was created and the knowledge of both carried on into more modern cultures to be picked up again.
Literature is the second factor that plays a huge role in why there are so many different pagan pantheons. While many different pantheons today can be traced back to ancient cultures like the Egyptians and the Greeks, many new pantheons today have been created in the fictional works of various authors over the years and new Mythos have been created. Using Literature is a great way to pass down old beliefs, legends and ideals but it has also allowed a good deal of creative influence on how the information has been passed. It also allows for it to be updated with the times for future generations and in the mix sometimes new ideals have arisen from older tales so even more Gods and Goddesses were created. This is also an opportune way for elders to pass on morality lessons to younger groups and over time those simple bedtime stories meant to frighten children into behaving have become real in the minds of that local group.
The final reason there seem to be so many different pagan pantheons is because of a growing number of groups and individuals who find themselves unable or uncomfortable with accepting all the deities in one particular sect and have begun to pick and choose among what is available. This is not done out of disrespect, but used as a way to utilize what is needed most in the individual or groups lives and calling on who they feel most adept in that area.
In our daily lives we call on the people we feel most qualified for the job we need them to do, be it the best plumber or the best doctor or landscaper, the same goes with building the best pantheon. For instance, someone may follow a general over all God and Goddess, but pray to Hermes specifically when they are travelling. This is not a slight against their deity, but simply calling on someone who has a sort of expertise on travelers. So when you ask why there are so many different pagan pantheons, you can just take a look around you at how diverse our planet is to get your answer.