Could be that until the SCOTUS decision, gay marriage went unrecognized on a federal level, with state exceptions. To boycott the states that didn't individually recognize gay marriage would be hypocritical without boycotting the federal policy that allowed those states to function that way. Now the mandate is reversed. Nothing in federal law or the constitution suggests that trans people ought to be discriminated against in this way or any way- nothing exempts them from enjoying the same civil rights as everybody else. So if a state chooses to craft a law as NC has done, it stands counter to current federal policy. Much easier to boycott un-hypocritically.
Could also be that gay marriage sent a clear signal re. popular opinion. Polls are pretty unambiguous- a majority of folks support gay marriage; support from the right is behind, but growing. Public response after Obergefell demonstrated that more tangibly. In the aftermath, businesses as well as other political actors may feel more comfortable signalling support on a range of LGBT issues (I know "gay" and "trans" issues ought not not always be conflated, but I'm not sure how that nuance translates in the larger political climate).
And- totally speculating- but could be something about the nature of this law in regards to state policy countermanding local policy...? Maybe if a company sees that a state is willing to shit on local ordinance, that gives them reason to believe that future application of such policy could further hurt local operation of business. Dunno.
Could also be that businesses have used their clout to boycott similar issues in the past, but didn't get as much publicity. Could probably be looked up, but I'm on my phone and anyways it's time to get off the toilet and make a sandwich and study.