Not all that surprising, really.
Uniting in opposition doesn't have the same negative impacts on people - on their constituents - as uniting to change or abolish a federal program that benefits people.
In opposition, uniting to block changes - regardless of the merit of the changes - preserves the status quo. Voters are less likely to be upset by a continuation of the status quo than they are by the elimination of a program that benefits them. This is true even if the proposed change that is blocked would benefit them, because many of the voters won't understand the benefit they would gain; many voters just don't pay that much attention to proposals; they'll never realize that they would have been better off if some program had not been blocked. But kill a program they're already benefitting from? Most of them will figure out what happened.
Most of the Republicans in the Senate will understand this, and understand that Graham-Cassidy threatens their constituents (voters in their states). But... to the extent that their seats are "safe" (e.g., they'll get re-elected even if they harm their constituents, because of large party majorities back home, coupled with "pull the big lever" voting), they can choose to ignore their constituents' welfare and focus instead on what their major donors want (the Koch, Adelson, Mercer, Scaife, etc. bunch).
We can hope that enough of the Republicans will vote sensibly - in the interests of the people at large, rather than just the big donors. Collins, Murkowski, McCain... Rand Paul? We'll see.
The potential silver lining here, if the Republicans do succeed in passing Graham-Cassidy: the resulting chaos may actually tip Congress back to the Democrats in 2018 or 2020 - and if that happens, coupled with the ongoing chaos that Grahan-Cassidy would cause in the health care industry, we might just finally be in a position to create a truly universal system in this country.