Who is the man?
They just call him...the Dr...
And on to the actual OP
As others have said fashion, trends, styles. Partypony is your gal for how fashion affects our culture and vice versa but at the end of the day any product which is dependant on culture.
The style of ponies suits the style currently popular - even if in directly. So as the years pass and trends change so ponies and their packaging change.