NIH had originally proposed making this mandatory years ago. The date got pushed back (possibly more than once). The first "open access" mandate was over a decade ago but exceptions were carved out, and there were also some very long grants that were already funded that were exempt from new regulations. This is just the latest - and hopefully last - step.
The foot dragging came more from the journals than anything. It wouldn't be that hard for Nature, Science, Cell, and the rest of the most prestigious journals to just make all the new articles available for all; they still make plenty of money on review charges, page fees, subscription fees, and fees charged to people who don't reside in the USA. They just kept finding excuses and their prestige kept people from fighting back harder.