From Wards Auto: Weak Dollar Limits BMW NA Options:

The continuing weak relationship of the U.S. dollar to the euro is adversely impacting importer BMW of North America LLC’s margins, CEO Tom Purves says.
Although BMW AG has not reduced North American allocation as a consequence, BMWNA is unlikely to see increased shipments of imported vehicles, despite this year’s introduction of an all-new 3-Series – the auto maker’s volume model.
Thanks to dollar’s weakness, North American sales of imported BMWs generate less profit.
“If we get an increase in production, we wouldn’t see an increased volume for the U.S.,” Purves says. “We would raise allocations for areas where the currency is stronger than the dollar.”
The U.S. remains BMW’s top export market, with sales of about 260,000 units. The U.K. is next with about 105,000 units annually

While this story primarily discusses the BMW models, the same is very likely to apply to MINI; I have already heard of a lack of Convertibles through the summer months.