Well, many strains of syphilis are becoming resistant to standard courses of antibiotics, although I don't think it's quite accurate to say that syphilis is "immune" to antibiotics. That's certainly a possibility in the future, though.

Part of our problem in the United States right now with syphilis is lack of affordable health care. When people feel they can't afford to go to the doctor, then they can miss routine testing. It's my understanding that initial symptoms of syphilis (genital blisters, etc) can sometimes be overlooked or dismissed as not very important. Then they go away, and syphilis enters a years- or decades-long dormant stage before reemerging to do pretty horrific systemic damage.

So when people miss or brush aside the initial symptoms, they're going to be in a world of hurt if they don't go to the doctor and get routine testing.

Americans' access to routine health care is on a steady decline. Going to the doctor is becoming more expensive, even for people with good insurance. Hell, I avoid going to the doctor if I don't feel that I can afford a $60 co-payment (up from $50 not long ago), which is not chump change for me. And increasingly, doctors want that copayment up front. (Often because they've voluntarily or otherwise become part of venture-capital funded practices that are very strict about maximizing billing and profits.)

The poorest of the U.S. poor can generally access reasonably decent (though not top tier) routine health care, if they can manage transportation. But the working poor are in a very different boat, and it's the working poor who are experiencing a surge in syphilis rates.