Low prestige, low pay relative to specialties (meaning non-primary care), mounds of paperwork (more than other fields on average), a relative excess of residency spots, etc. There are more reasons but I can't think of them atm.
Personally, I would hate to manage cholesterol and diabetes every day. Sounds extremely boring. Plus, the knowledge base is way too broad. I would rather know one thing extremely well than know a lot of things well enough to pass a pt off to a specialist.
Edit: I will add that those offers definitely aren't a recent trend. I will say that you may be looked down upon (stupidly so) by your classmates for choosing primary care because there's no glamour associated with it and it's what people that didn't do well on step, have red flags, etc match into. This is not to say that there aren't plenty of people that are highly competitive that choose primary care. It's all about what you want at the end of the day. Who cares what anyone else thinks. It's your life.