Probably.
And also probably because Heller voted for Rubio’s Gang of Amnesty bill.
If he sided with the citizens and the rule of law, he probably wouldn’t be in trouble.
Heller is vulnerable as he is running a blue state Trump did not carry and Obama easily carried twice. That bill isn’t even a topic in that election. Although yes, the bill was terrible.
Although it would be nice if your logic did hold - we would have 75 senate seats if all of the Democrats lost this year who also supported that bill - would gladly sacrifice Heller in exchange for gaining all of those other seats. However, in this case, that isn’t going to happen so we need to hold as many seats as possible and pick up at least a few - otherwise you-know-who is about to replace Hatch and the smaller the majority the more clout he will personally have in the Senate. There was discussion of the need for a net gain of 2 to counter Collins and Murkowski - but we really need 3 to also counter Romney.