From a quick internet search, I notice that not even Japanese monks seem to know the real answer to your question. But here are examples from the many theories they have provided.
Why budda statues in general are often not to be photographed
http://hasunoha.jp/questions/252Why we at Kaizenji decided to prohibit photography
http://kaizenji.org/satuei.htmlI can't translate all of them, but here is a gist of what they are saying:
"I've always asked that question to myself as a monk. Temples often gave me answers like 'the flashlight ruins the statues,' which didn't really make sense to me. But I suppose it's more important to experience yourself
being there. This is a religious
field where you are expected to be the player rather than the viewer."
"I think it's more to do with how the photos are handled. These are statues with a lot of history of many people's spirits and wishes. If you are a priest, you would handle the photos with care by perhaps adding a few words to it so that the viewers' souls would be religiously saved by them. But suppose a stranger randomly uploaded your deceased parent's precious belonging on his/her blog without permission. What would you think?"
"While a budda statue is something to be honored and worshiped, taking a photo of it may often imply that the statue is a
plain object to you. That said, we welcome photographers at our temple."
"We used to welcome amateur photographers at our temple of Kaienji. I don't believe that there is anything here to be restricted from being photographed.
But unlike the past, when cameras were valuable tools available for a limited number of people that had earned special skills, today is the era of the digital camera. Even I switched to digital since I could no longer find films being sold. It's great that more people now have the opportunity of taking better photos so easily.
But since taking photos has become something so easy to do, I notice that more people now often tend to get so enthusiastic that they
take an extra step further. For example, you may want to take a close-up of a flower, but getting the lens too close may not always be good for the plant life. And a tripod may get in people's ways. Then the people trying to walk around that tripod may feel the need to take that extra step further into a place where they are not appreciated to be."