Because, other than on TB, most people don't use them, and wouldn't like the additional tension, and because the bass would need a new setup. Also, because they are strung with rounds at the factories, and it is silly to change them for minority players. You play for the odds, and way over 90% of bassists use rounds. Mostly, though, it's money. Whenever a question starts with, "why don't they..." The answer is almost always, "money." In this case, rounds are a ton cheaper, especially when they are purchased in bulk, and you only need to do one setup instead of having every bass strung differently, and thus needing a different setup.