[A_Brandybuck]

I think, that it is worth considering the fact, that the Nazgûl were abroad and deep in the 'enemies land'. Although they are very powerful in spreading fear, they had to be careful not to attract much attention, because there are enemies, which are more powerful then they are.

Taking Glorfindel as a example, who drove four Nazgûl from the Bridge and Gandalf was still 'lurking around'. There was a bunch of 'terrible' High-Elves in Rivendell and many Ranger in the wilderness.

And before weathertop, the Nazgûl weren't sure about the bearer of the Ring. That is, in my meaning, important to know to understand the behaviour of the Nazgûl in Bree. A direct attack could fail, if the bearer has been very powerful and could use the Ring. Especially because a Ranger with a powerful aura is with them.