Because we had more pressing needs. Last year we needed to re-sign Tanne and get Henry a new contract. And to go along with that, we needed to improve our pass rush. So we spent pretty much all of our money on 4 players. The plan was to play Kelly at RT for a year then have Wilson take over in 2021. Obviously Beasley and Clowney turned out to be terrible signings, and Wilson ended up being the worst draft pick we've ever had. But JRob didn't re-sign Conk because he wasn't good or that he didn't want him. He simply had a different plan to improve the team that didn't work out.
This year, the cap was reduced and we had a lot of holes to fill with not much money to do it. The decision to not pick up either player's 5th year option is due almost entirely to injury and needing the cap space. If Davis had been healthy in 2019 and performed this way he did in 2020, there is no question they would have picked up the option (though he might have still been cut given the reduced cap). If Adoree was a little more healthy, he probably doesn't get cut either, seeing as he was very good when healthy.
Again, most player's don't re-sign after their rookie contracts. That doesn't have anything to do with whether or not they were good picks.