In Nashville, you have a very well-oiled machine in terms of the studio and demo. system. Songwriters don't know who is going to cut their songs when they are written. The artist may be male or female. Producers and record labels have a huge say in the final recording as well. So a song may be recorded in a different key than that which it is was demoed. Putting numbers on the lead sheet made it easier for the session musicians, especially if a key was changed in session.
Professional musicians in America are often not familiar with solfege. The chords would be written out in letters and changing the key is easier having them written out in numbers via the Nashville number system.
With all theory, it is important to remember the the music came first, before someone explained it theoretically.
