Why had Paul been hindered on several occasions from visiting the saints in Rome?...

Because of the time he was spending going where Christ was not known.

The people of Rome already had heard the Gospel. Citizens of that city had been in Jerusalem observing the feast of Pentecost when the Holy Spirit first descended on the Christian Church. Also, it is believed that some Christian people who had heard the Gospel from Paul’s mouth had gone to live in Rome (see Chapter 16 of Romans).