It is (kinda). See OC 155 and 156.
Additionally, I speculate that even though Talmud Torah must be done every day, its nature is not that of a 'time-bound mitzva' in the sense that the mitzva is ideally והגית בו יומם ולילה to delve into it day and night. It is a all encompassing mitzva that spans one's whole life and whose goal is never really reached. This is very different from something like kriat shema or lulav which have very specific yes's and no's for specific times and instances. The regularity aspect of Talmud Torah is indeed quoted in Orach Chaim above, but the general obligation is reserved for Yore De'ah along with other general life mitzvot.