To make animation appealing you need to anticipate, exaggerate, and squash/stretch motion. These do not appear on rotoscoped footage because they do not occur

To make animation appealing you need to anticipate, exaggerate, and squash/stretch motion. These do not appear on rotoscoped footage because they do not occur in real life. If you rely on rotoscoping the result is bland movements which lack a "spark of life". In other words, rotoscoping creates an "uncanny valley" for motion.