Several good and reasonable reasons have been identified here. But one that hasn't been mentioned that also contributes to this is that Canon is a multinational company. Its costs cannot be limited to any one nation or one currency.
A falling Yen does not reduce the costs of its production facilities in other countries and it may actually increase some of its costs for materials and labor.
Another major consideration – Advertising makes up a huge percentage of a company's total costs. A falling Yen is not going to reduce the cost of advertising in National Geographic or any of the hundreds of publications that Canon advertises in. They have to pay for those ads in the local currency, so exchange rates, again, can hurt as much as help.
When talking about a company the size of Canon, there are just too many variables to presume that one thing (like the exchange rate of the Yen) will have a major impact on prices