For Paul, belief in the resurrection was essential for salvation. But in this case he did not formally insist on it, perhaps because he did not wish those who did not confess it to be excluded from the community. (Historically, it was not until the second century that the church excommunicated those who did not confess faith in the physical resurrection. Moreover, according to Acts 6:7, a large number of priests had joined the faith, and since priests tended to be Sadducees who did not accept the doctrine of resurrection of the dead (Acts 23:8), we may presume that some of them denied the resurrection while still confessing that "Jesus is Lord.")
However, it is clear that the resurrection was a central doctrine for Paul. In his theology, all humanity was under the curse of death; and it was necessary for Christ to die and be resurrected in order that humanity could attain eternal life:
1 Corinthians 15:21-22
21 For since death came through a human being, the resurrection of the dead came also through a human being. 22 For just as in Adam all die, so too in Christ shall all be brought to life.
Paul also taught that active belief in Christ's resurrection is an essential part of Christian faith, without which the hope of salvation is vain:
1 Cor. 15
If Christ is preached as raised from the dead, how can some among you say there is no resurrection of the dead? 13 If there is no resurrection of the dead, then neither has Christ been raised. 14 And if Christ has not been raised, then empty is our preaching; empty, too, your faith. 15 Then we are also false witnesses to God, because we testified against God that he raised Christ, whom he did not raise if in fact the dead are not raised. 16 For if the dead are not raised, neither has Christ been raised, 17 and if Christ has not been raised, your faith is vain; you are still in your sins.
Thus, affirming a belief in the resurrection of the dead was a central part of Paul's gospel. If the dead are not raised, then neither was Christ. As in Corinth, some members of the Roman church probably did not yet accept this doctrine. We do know that in later decades, the Roman church included Gnostics who emphasized spiritual enlightenment rather than actual resurrection. In Paul's own time, it is clear that some Christians did not accept the idea. As to why he didn't insist on also confessing it here, he didcame close to doing so in 1 Corinthians but stopped short of saying that those who did not confess it should be excommunicated. So I would conclude that, even though Paul thought that belief in the resurrection was essential, he did not think people should excluded from the community if they thought otherwise, as long as they did confess the kerygma: "Jesus is Lord."