Yep, Maryville is 865 now. One reason why so many area codes were added during the 1990s is the explosion of cellular, fax, pager numbers and so on, as well as increased demand for regular lines in general.
It was also the case that a whole prefix (the first 3 digits of your 7-digit number) had to be assigned to a specific central office or company. For example, if a small cellular phone system was set up and was assigned, say, 265 it would be assigned the whole block of ten thousand numbers from 265-0000 through 265-9999, even if only a few hundred numbers were actually needed. Multiply that over many such instances and you can see how a lot of numbers were vacant but unable to be used. These days, the system has been altered so that ranges can be assigned by the thousand block, so that same small company might get 265-1000 through 265-1999 leaving 9000 other 265 numbers free.
But the shift of population southward and westward over the years has certainly had an influence from way back. For example, in the original 1947 area code plan, Florida had just one area code, and even California had only three!