The reason is simple - the factories are constantly running, with a production schedule set up months in advance, which means they can't increase a production run by too much more than the quantity that is scheduled, as it would delay products scheduled to follow it... while a second production run might need to wait 2-3 months for the next available timeslot (if Hasbro felt that there is enough demand for a second run and retailers are willing to wait that long, as other stock would arrive before it anyway).
So, if a toyline is more popular than American retailers expected and want to significantly increase their orders too close to the next wave's release, the factories might have already finished production of that wave, or can't produce too much more to cover it, so the stock allocated to other countries gets redirected to America. Hasbro is an American company, and are not going to sacrifice their American business & profit (and retailer reputation) to make sure other countries get stock.
This is sometimes why we miss waves or exclusives here (most of the time it is because there is not enough demand here for extra products that aren't in the retail lines), but this year it looks like Hasbro America reduced all of their production numbers due to the global downturn in retail sales from the recession, but some rare items like the Netflix toys, ended up exceeding their new lower estimates.
Story time... :p
Several times at BotCon, the organisers noted a number of times that they wanted to increase the number of exclusives (
2011 had boxsets sell out in 6 days of pre-registrations opening up), but were often unable to, because 2-4 months before the event was too late to increase to increase production and too late to squeeze in a second production run to make up the extra numbers. Most years they would comment that their tight schedule with convention and club exclusives would often have the toys arrive just a week or two before the convention (time to be produced and shipped).
Back when Armada came to Australia in 2002, the first wave of the toyline was released in America in July, but came out in Australia in December.... and I was told by Hasbro Australia at the time, that this was due to the higher than expected demand for the toyline by American retailers, absorbed all of the stock, and we (and other countries) had to wait until a second production run was done 5 months after the first production run.
(according to Aaron Archer (I think, without looking it up) at BotCon one year, the Armada toyline was in such increased demand by American retailers to the previous year's toyline (RID), that we saw the back end padded out with a lot of redecos that weren't in the cartoon, including those Beast Wars moulds we saw)