I imagine any of the following could be possible:
1. He could still be not 100% whether because of his recent bout of the flu or something else.
2. Stevens thinks he is making too many mistakes, wants to address them in practice which they haven't had.
3. Stevens has more confidence in Green, Smart, Wanamaker, Waters, Brown and Kemba in the backcourt, making Langford about on equal ground with Carsen Edwards.
4. Stevens has more confidence in Brown, Tatum, Hayward, Smart, Grant Williams, Green, and Semi as a wing making Langford dead last in that rotation, too.
5. Stevens just isn't seeing enough of a work ethic off the court so feels Langford hasn't earned the time
6. Langford is being punished for being late for something.
7. Something else the coaching staff knows that we don't.
8. A combination of any of #1 thru #7.
It could all add up to he just isn't ready so rather than force him into the court and possibly hurt Romeo's confidence if he struggles, Stevens is just letting him learn and adjust to life as a pro. Not every young player has the same developmental curve. We need to be patient. Langford is just 20 years old, after all.