Why quarantine is preferred?

New strain of H5N1 bird flu emerges in China

...those vaccine-induced antibodies do not recognise the Fujian virus, although they do attack the virus strains that Fujian has now replaced.
This means the Fujian strain has a selective advantage in vaccinated birds. "This novel variant may have become dominant because it was not as easily affected as other strains by the current avian vaccine," says Guan. That may also be why H5N1 infection in Chinese poultry has surged, rather than decreased, despite increased poultry vaccination.

Worryingly, the antibodies being used to develop human vaccines for H5N1 ... do not recognise the Fujian strain. This means the current experimental pandemic vaccine would not work against any pandemic virus that emerged equipped with Fujian surface proteins.

In the face of mutating pathogens it's easy to see why monitoring and quarantine are favored weapons against epidemics.