herpes viruses & immune system

[Julian Brown]

Can someone explain how it is that the herpes viruses evade the immune
system?  I realise they integrate themselves into the genome and are
able to remain dormant.  But why is it that once they reactivate the
immune system isn't able to clobber the multiplying viruses?  If the
virus has been dealt with in a primary infection one might expect the
the immune system to be able to prevent secondary outbreaks.  Why
doesn't it?