The big reason? Construal theory, or as I like to call it, action is not an abstraction. Abstract construal doesn't prime action; concrete construal does.

Second big reason: the affect (yes, I do mean affect) of being precise, is very much negative. Focusing your attention on flaws and potential problems leads to pessimism, not optimism. But optimism is correlated with success, pessimism is not.

Sure, pessimism has some benefits in a technical career, in terms of being good at what you do. But it's in conflict with other things you need for a successful career. TV's Dr. House is an extreme example, but most real people are not as good at the technical part of their job as House nor are the quality of their results usually as important.

Both of these things combine to create the next major problem: a disposition to non-co-operative behavior, aka the "why can't our kind get along?" problem.

Yes, not everyone has these issues, diverse community, etc. But, as a stereotypical and somewhat flippant summary, the issue is that simply by the nature of valuing truth -- precise truth, rather then the mere idea of truth -- one is treating it as being more important than other goals. That means it's rather unlikely that a person interested in it will be sufficiently interested in other goals to make progress there. I would expect it more likely that a person who is not naturally inclined towards rationalism would be able to put it to good use, than someone who's just intellectually interested in rationalism as a conversation topic or as an ideal to aspire to.

To put it another way, if you already have "something to protect", such that rationality is a means towards that end, then rationality can be of some value. If you value rationality for its own sake, well, then that is your goal, and so you can perhaps be called "successful" in relation to it, but it's not likely that anyone who doesn't value rationality for its own sake will consider your accomplishments impressive.

So, the truth value of "rationalists don't win" depends on your definition of "win". Is it "win at achieving their own, perhaps less-than-socially-valued goals? Or "win at things that are impressive to non-rationalists"? I think the latter category is far less likely to occur for those whose terminal values are aimed somewhere near rationality or truth for its own sake.