Because I'm American (and I like the Europeans a lot, too ;~). No, really.
For the US and Europe, the relative values of currencies not only determine what we pay for our camera equipment, but also directly impacts the profit that the Japanese companies make. While I try to avoid political discussion on this site, the US government's old policy of letting the dollar stay weak against other currencies to try to encourage US exports definitely had impact on all of us. Nikon, for example, in their 2011 fiscal year reported that the appreciation of the yen (against both the dollar and Euro) had a net impact of -27 billion yen in their profits.
These days, the trend is somewhat reversed. The appreciation of the dollar against the yen when Abe took over increased Nikon’s profits, as they didn't lower their prices as fast as the decline in the yen in the 2014-2015 period.
If you build something but by the time it actually sells overseas it brings in 5% less than you expected due to currency fluctuation, your gross operating profits go down. If it brings in 5% more than you expected due to currency fluctuation, your gross operating profits go up. Overall, we photographers want a stable yen/dollar or yen/Euro relationship, as it means the companies we count on continue to be profitable and the prices we pay for our equipment don't change or get out of hand.
So, if you're trying to figure out how things are going, look up the current currency rates and compare them to these: 110 yen to the dollar, 120 yen to the Euro. Those are the figures that Nikon is currently using to predict prices and profits. If those numbers go up or down significantly, we'll eventually see price changes.