Jacqurine,
i also have asked the same question to many PGs and VGs...for most PGs that i have talked to, many of them only work with certain VGs...then for VGs, they have some PGs that they work with too...
the reason given by PGs is when they are photographing a moment, some VGs sometimes get in the way...
then for VGs, they give the reason that videos are continuous, for example with the covering of the veil...some PGs make the parents stop for a moment while covering the veil...for the VG, that would kill his video...and if he makes the parents and bride repeat that moment again, the feeling and emotion will be lost...