According to a note from widely followed KGI Securities analyst Ming-Chi Kuo that was obtained by MacRumors, Apple's suppliers are only able to build 10,000 iPhone X units per day. Kuo said this "means the model will remain in severe short supply for a while."
This aligns with earlier reports, including from Kuo, that suggested Apple was having a difficult time manufacturing the phones. One constraint might be OLED panels, which are in short supply. UBS said in August that the high-end iPhone would be delayed until September, so it sounds like Apple has managed to work out some kinks.