The Apostle Paul struggled for legitimacy. That is evident from Galatians, Philippians, and 2 Corinthians. Some Jewish Christians contested Paul’s apostolic credentials and the validity of his proselytism-free gospel. So when Paul writes to the Romans, with a view to visiting them, it is a risky venture. He knows that his reputation was subject to misrepresentation (see Rom 3:8). Would the Roman Gentile Christians recognize him as their apostle? Could he count on their support for a mission to Spain? Would he able to return to Jerusalem with ALL of the Gentile churches behind him? There were no certainties here.
Ernst Kasemann picks up on this point in his comments on Rom 1:13 where Paul explains why he hasn’t visited them:
The most important theological epistle in Christian history is undoubtedly also the record of an existence struggling for recognition and of an apostolicity called into question. Apart from this insight Romans cannot be interpreted correctly.
A true point, which is why Paul feels the need to explain why he has not visited them before, and why he is less polemical in his discussion about the Mosaic Law than in Galatians.