As mentioned already it's likely because ETH and Ole have not cared about holding onto possession and controlling games through passing. ETH has literally said about wanting us to to be the best transition team, says we can't play like Ajax and has regularly rebuked criticism like our poor possession percentages and shots conceded. His tactics and team selection is all about 'big hitters' e.g take-ons, hard running, physical presence, duels, pressing high etc. It's not conducive to play more possession, pass and move football the likes of Amad, Hojlund and Mainoo can excel in or most of the team for that matter.
I don't think there's a single reason as why Amad hasn't been getting minutes. I just think ETH's general incompetence, stubbornness with regards to tactics, giving players an extremely wide berth to find form etc and all his general inconsistencies like his preference for seniority on top of our injuries means something has gone amiss.