I would say that first of all both are very marginal hands from MP, and even the hand that does open is only winning the very minimal possible by opening.
That being said, I would say most likely its because the 2x in our hand will generate slightly more value than the 6x in this instance. Our opponent is less likely to put us on 2x than on 6x here and therefore we might gain a bit more with that card, as our opponent might invest more if that card hits the board as we will do worse there with our range.
It is not because of the 2nd nut straight as you mentioned here, as having QJT6 will than give you the nut straight on 789, having JT.
Overall its hard to conclude exactly whats happening and why, but I have seen similar outcomes where the solver prefers lower cards (2/3/4) over more middling cards (6/7).