The most charitable interpretation would just be that there happened to be a convincing technical theory which said you should two-box, because it took an even more technical theory to explain why you should one-box and this was not constructed, along with the rest of the edifice to explain what one-boxing means in terms of epistemic models, concepts of instrumental rationality, the relation to traditional philosophy's 'free will problem', etcetera. In other words, they simply bad-lucked onto an edifice of persuasive, technical, but ultimately incorrect argument.

We could guess other motives for people to two-box, like memetic pressure for partial counterintuitiveness, but why go to that effort now? Better TDT writeups are on the way, and eventually we'll get to see what the field says about the improved TDT writeups. If it's important to know what other hidden motives might be at work, we'll have a better idea after we negate the usually-stated motive of, "The only good technical theory we have says you should two-box." Perhaps the field will experience a large conversion once presented with a good enough writeup and then we'll know there weren't any other significant motives.