Answer:
Having worked as a Music Supervisor with ad agencies for over 30 years, I would say most often it is down to budget/not having any money. Classical music is in Public Domain(PD) which means no Publishers fees and many library catalogues have recordings of classical music = synchronization fees at the lowest fixed rates. It is important however to check if the track is in PD or the version that you look is not a new arrangement it can turn out to be an expensive error.