There should be an equilibrium between security and ease of use (convenience) so that users feel comfortable playing around with the system. When you add more security to your system, it makes it too heavy and complex, which in itself may lead to unexpected errors and hard-to-find vulnerabilities. In other words, complex systems are not only inconvenient but also very hard to maintain. Twenty-four-word seeds are undoubtedly more resistant to random attempts of brute forcing, but at the same time, it is sometimes a tough task to back them up properly. Moreover, the private keys produced from such lengthy seed phrases can't provide more than 128 bits of security, which makes the whole idea of using very long keys less attractive. Twelve-word seeds are shorter, easier to back up, or even memorize (I don't say you should try to memorize them, but it is good to have such an option). In other words, while short seed phrases are more vulnerable to brute force, they generate private keys of the same length and, at the same time, are more convenient when compared to heavy seeds.