There's always going to be a tradeoff between security and speed/usability. If they implement an Iris scanner which would take 30+ seconds to unlock, or will have a much lower reliability rate when in the dark/wearing glasses, then that would be a worse feature due to its hindrance, even if the security itself is better.
Currently the security on Face ID is good enough and the motion of unlocking becomes second nature. Certainly as the technology gets better and faster, it could be feasible to have Iris scanning which detects and scans very quickly. At the moment I'm not sure it would be up to Apple's high standards.