Because global variables can be modified by the process without other parts of your code knowing it producing unexpected results.
You should always try to keep variables scoped as locally as possible -- not only will this help you with debugging, but it will keep your code easier and cleaner to read and go back and modify.
If you are looking to share data across multiple functions, you might look into making a class for your data and then defining methods to operate on the data encapsulated in your object. (Object-Oriented programming)
