International to international transfers do not exist in the USA (who knows
why!) hence the need to clear border control and customs, pick up bags, go
upstairs (most of the time), check in, go through security, then back to the
gate. If you're lucky, your bags may be checked through, but the same palaver has to be gone through.
AFAIK it's always been so
Miami allows for intl-intl transfers without the need to collect your bags and allowed to stay airside.