Over time, even as fewer literate people knew Latin, the "b" was kept because it marked important, meaningful connections to other related words, like "dubious" and "indubitalbly," which were subsequently borrowed into English from the same Latin root, "dubitare".
Con el tiempo, aunque menos alfabetizados sabían latín, la "b" se conservó porque marcaba conexiones importantes y significativas relacionadas  con otras palabras, como "dubitativo" e "indudablemente", que fueron subsecuentemente prestadas al inglés de la misma raíz latina, "dubitare".