Easier to stack and move, don't have to worry about a bag rotting/splitting and spilling the coins all over the place etc. Easier to count if you are using them for payments between countries. (Here's two gold bars, instead of having to count 2500 or whatever gold coins. Because you know the receiving party is going to want them counted.)
Close to true if you are using double eagles. 800 coins for eagles, 1600 for half eagles, and 3200 for quarter eagles. so for 2 bars and a mixture of coins a guess of 2500 probably isn't too far off.
When the post-War hyperinflations and upheavals began, the U.S. coin was seen as the standard bearer. In times of trouble, folks want U.S. assets -- or U.S. gold. I'm not surprised that so many 1920's found their way overseas compared to other vintages. I'm sure there could have been other reasons, too.